Add user/workshop/curator follow & unfollow support

[3urobeat]

Hey, it's me again.
This PR adds support for following and unfollowing a user.

Following a user from their profile dropdown or from the user's "Workshop Items" page does the same thing, that's why I mentioned both in the title.

 

Documentation for the CSteamUser wiki page:

### follow([callback])
- `callback` - Optional.
    - `err` - `null` on success, an `Error` object on failure

**Non-object method name: `followUser`**

**v3.x.x or later is required to use this method.**

Follows the user and their workshop.

### unfollow([callback])
- `callback` - Optional.
    - `err` - `null` on success, an `Error` object on failure

**Non-object method name: `unfollowUser`**

**v3.x.x or later is required to use this method.**

Unfollows the user and their workshop.

Don't forget to fill out the version number should you copy and paste this.

 

That's it, I hope the amount of PRs by me don't get on your nerves too much (warning: there is probably one more coming soon because I want to add support for reviews).
Have a nice day :)

[3urobeat]

I have also added support for following & unfollowing curators; it uses a different endpoint.

I added the two functions to groups.js as curator pages are made from groups and are tied to another - I hope this makes sense; I did not want to create a separate file for two methods.
I did not add object methods to CSteamGroup.js as the clanid param used in this request is different from the gid or any other group related id.
Please let me know if I should rather create a new file for the two methods.

Documentation for the SteamCommunity wiki page:

### followCurator(clanid, [callback])
- `clanid` - ID of the curator, is displayed in the URL
- `callback` - Optional.
    - `err` - `null` on success, an `Error` object on failure

**v3.x.x or later is required to use this method.**

Follows a curator page.  
Note: Requires an unlimited account.

### unfollowCurator(clanid, [callback])
- `clanid` - ID of the curator, is displayed in the URL
- `callback` - Optional.
    - `err` - `null` on success, an `Error` object on failure

**v3.x.x or later is required to use this method.**

Unfollows a curator page.  
Note: Requires an unlimited account.

Don't forget to fill out the version number should you copy and paste this.

To quickly test this, call the function with the clanid 1850 to follow the well-known PC-Gamer curator page.

[Revadike]

You prob want to merge it into #230

[3urobeat]

Yeah I thought about that but ultimately decided against it as DoctorMcKay merged my previous PRs into v4 afterwards anyway and the v4 branch seems a bit stale.
I can open another PR/rebase this one though if you wish

[DoctorMcKay]

Is body.success a number? If so, it might be an eresult rather than boolean.

[3urobeat]

You are right, this should be an eresult. I'll fix this real quick

[3urobeat]

Should be correct like this.
Steam returns for users: { success: 1 }
...and for curators: { success: { success: 1, nSaleTaskCompleted: null } }

[DoctorMcKay]

Helpers.eresultError does all this for you.

[DoctorMcKay]

Let's target this at master.

[3urobeat]

Like this? There are quite a few functions which do not use the eresultError() helper...

[DoctorMcKay]

Yeah, looks good. I'll merge & deploy this later tonight, once I can get some docs written.

There are quite a few functions which do not use the eresultError() helper

Those predate the helper, most likely.