Add `Tilesets` API

[igor47]

I am re-opening mapbox/mapbox-sdk-py#236 -- i'm guessing that @critical-path just gave up waiting for that PR to be reviewed and merged. @critical-path -- thanks for the code.

Given that this repo is not active right now (and so this PR is unlikely to be merged), if you (like me) need this API in your project, you can install it like so:

pip install git+https://github.com/Aclima/mapbox-sdk-py.git#egg=mapbox

after you pip freeze, replace the line in your requirements.txt that says mapbox==0.18.0 with a line that says -e git+https://github.com/Aclima/mapbox-sdk-py.git#egg=mapbox; the resulting diff:

-mapbox==0.18.0
+-e git+https://github.com/Aclima/mapbox-sdk-py.git#egg=mapbox

this PR closes mapbox/mapbox-sdk-py#152