Fixing bad URL formation when getting all commits

The method was using the SourceCodeURL attribute from the Project to build the URL, but sometimes this attribute may be not present, giving a NullPointerException.
Fixed test for get all commits method
Enabling fetching of issue labels as requested by Irineu (@imlm). Closes #65
Updating README
Fixing persistence issue with Commits due to the lack of a primary key in some commit-related objects. Related to #63
Fixing Milestone fetching
Minor cleanup
Finishing DB coverage for all entities in Groundhog, except for the 'irregular' which are License, Language and IssueLabel. Related to #63. Now, only the test coverage for this feature is missing to the issue to be closed.
Update README.md
Update README.md
New class that represent a Contributor in a Project.

A class that can store all information about a Contributor from a
project like we talked about in Issue #68.
Created a new class that contains all tests of a basic Github entity class.

I wanted to test all basic behaviors on basic classes and i didn't found
a class to this, then i created a new one.
Change the "equals" method on User class.
Fixing issue causing Issue.isOpen() to always return false
Both the condition and the equality function used were wrong, the code
expected to return true if the issue's current state was "closed"
instead of "open" and instead of using String.equals(..) it was using
the reference equality operator "=="
The current implementation will return true only when the issue current
state equals "open", this means that it may return false even for states
that are not "closed" (e.g., null)