API key handling

[RenoFilla]

Previous API key implementation was faulty and unusable: for private channels there is a difference between User API key (for modifying a channel or viewing its setup), Read API key (reading data) and Write API key (pushing data).

[mchwalisz]

I'm aware that there are multiple kinds of keys, but AFAIK they have the same format and it's not necessary to separate them on the API level. Your implementation makes it harder to write code. It's a bit more explicit but IMO it's easier to have one api_key variable and feed it write key if we want to write data (reads should work as well) and read key when we are only interested in reading data. In the second option writing data will raise an exception, which is IMO fine.

Please, tell me if anything from the above is wrong.