Allows to define page size for repository lists and other API enhancements

Extension of the listRepositories() with the desired
pageSize. This allows to reduce the number of calls
to GitHub API for fetching the entire set of repositories
browsing all the pages.

Additionally allows to match the UX paging with the
underlying GitHub API paging, increasing performance
and reducing hourly API allowance.

Last addition was on the GitHub PullRequest object:
allows to get the associated commits with their
details (which is unfortunately NOT a GHCommit object)