Changes related to #39, and more tests.

Mainly changes in the `tldextract.TLDExtract` callable-class, related
to making the new flexibility simpler... notably:

* Simplification of keyword arguments.
        * `suffix_list_url` for both files and public suffix list url;
        remove `Fetch`. (if you don't want it to fetch, set
        `suffix_list_url` to `False`.)
        * `cache_file` can now be set to something that evaluates to
        `False`, in order to disable caching.
        * Remove unnecessary `debug` keyword argument.
* Only complain about 1 combination of keyword arguments, where it's
necessary - and complain on construction, not calling.
* Update docstring of `tldextract.TLDExtract` accordingly

Add more tests: use more combinations of keyword arguments to make sure
everything still works in the different configurations.