Fix sort parameter handling

[timwedde]

The sort parameter is mishandled in the API class in that it is entirely ignored, leading to it becoming part of the search dict. This, in turn, makes it non-functional. This PR fixes this in the same way that the result_limit property is handled.

In the current implementation, the resulting param set looks like this:

{
    "id": -1,
    "method": "Get",
    "params": {
        "search": {
            "sort": {"sortBy": "date", "sortDirection": "desc", "offset": None, "lastId": None},
            "deviceSearch": {"id": "<myID>"},
        },
        "resultsLimit": 3,
        "typeName": "DriverChange",
        "credentials": <removed>,
    },
}

[aaront]

Fixed in #127. Thanks for your PR!

I ended up making a few more improvements and extracting the parameter handling into its own file.