Moving the "find" ability to entities instead session.

This will be necessary when implementing "Branch" entity since it differs from other entities on how results are obtained.
Adding support for loading additional information returned by Travis API.

Such additional information are set into main object being loaded.

Also creating Commit object (which is the most common information being returned together with other main information). This is important since there is no other way to obtain commit information.
Adding "Branch" entity.
Minor fixed just to make sure TravisPy will work on all platforms.
Trying to get coverage.
Removing unnecessary code.
Merge pull request #7 from menegazzo/fb-6-branch-entity

Branch entity