Deserialise JSONCommit with Gson

Rename changedFile object to FileInfo to match Gerrit documentation
Added CommitInfo to store patchset-specific details
Added FileInfoList to help with deserialisation
Added ReviewerList to help with deserialisation
Changed value in Reviewer to integer (it is not a string)

Change-Id: Ibbf8267c6612047cba51aec29a5346d6210d94fe