added PublicAPI class and etc

[allerter]

This PR makes the following additions/changes:

• Adds the PublicAPI class which covers the public endpoints available at genius.com/api. I tried to find as many of these endpoints as possible and to make sense of their parameters and responses. But since they're undocumented, can't be 100 percent sure.
• renamed all the methods with a get_ as an experiment to see how they'd look without that. But if you think it's better to leave the get_ part be, I'll easily revert it back.
• As there are now two API classes each with its own _make_request, I created a Sender class to be their base. Also, the API and PublicAPIclass can be initialized on their own like before, andPublicAPI` doesn't need a token.
• renamed _scrach_lyrics_from_url to lyrics to allow users to use this method by passing either song URLs or song IDs.
• reformatted the Artist object to allow for a client attribute. With this change, now the artist.song (formerly artist.get_song) is implemented (now it only gets a song and doesn't add it) and artist.add_song supports song names beside Song objects.
• updated some of the dependencies' versions and changed .travis.yml and tox.ini to allow for all tests (unit tests, code style tests and etc) on Travis CI.

[allerter]

@johnwmillr, thanks for reviewing and merging the PR. A question: why not squash and merge?

[johnwmillr]

Just a total mistake on my end, I'm afraid