Improving README a little bit
Creating a representative class for the Label objects present both in
the Issues and Milestones objects.

Currently, Groundhog supports no way for fetching these objects, but
this should come in briefly.
Fixing toString() method error that was ocurring to the Issue (and other
objects) related to the getURL() method of these respective objects,
just as reported by Irineu (@imlm).

This happened whenever those objects were fetched by their search
methods from the SearchGitHub class. This was due to the fact that the
fetched objects from the API were not having their Project assigned.

This way when methods like toString() (which themselves call other
methods like getURL()) were called, the chain was broke because of the
missing project, throwing java.lang.NullPointerException exceptions
everywhere.

Objects such as Commit, Release, Issue and PullRequest should *always*
have their project dependency filled. All of these objects only make
sense when they're related to a Project, because they're
project-based-only objects.

Thanks @imlm.