First implementation of the Issue entity, which is going to be much requested for our analysis on GitHub data.

This entity is going to rely on other entities which are not currently defined. Hence the Issue entity itself is not a 100% complete.

As requested on #38
Moving new entities to appropriate location in the source code
* Creating the PullRequest entity with all its relevant properties from the GitHub API.
* Adding relationship with PullRequest in Issue.

This issue is related to #38, for Pull Requests are no more than just special Issues (in terms of the GitHub API).
Closing #42
Improving toString methods