Starting work on Gson object attributes serialization

Sometimes the attribute name in the API differs from the attribute name in its correspondent class in Groundhog. Using SerializedName we can manage to solve any issues regarding these differences

More info at http://google-gson.googlecode.com/svn/tags/1.2.3/docs/javadocs/com/google/gson/annotations/SerializedName.html

Related to #6
Removing useless, redundant attribute from Project: followersCount. This attribute duplicates the watchersCount one.
Related to #6
Adding SerializedName annotations to PullRequest. Related to #6
Informs the creation date of the Milestone and adding updateAt attribute. Related to #6