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.
Added Flapdoodle's embedded [1] mongo as a dependency. Related to #63

This dependency provides a mongodb embedded version, specially used for unit testing.

[1]: https://github.com/flapdoodle-oss/de.flapdoodle.embed.mongo
Added a constructor that receives a MongoClient instance. Related to #63

This constructor is useful when we're using Mongo as an embedded database.
Added support for Mongo as an embedded database. Related to #63

This code configures an MongoDB embedded database on localhost for testing purposes.
Change all equals test on Entity Tests.

Assert.assertEquals(Object a, Object b) use operator "==" as comparison
and this only  compare the memory address.
"getAllContributors" method returning Contributor List and changes on test.

Contributor Class starting to be applied on "getAllContributors" method
and reapplied on respective Suit Test.
Project test and annotations(documentations).

Created more tests that protect all behaviors expected in GitHub
Entities pre-defined by other Groundhog contributor which maybe can be
affected by some code change in basic classes.
Improved Project anotation.
Just removing a unnecessary comment line.