Methods of HistoryEntry which directly work with the File System have been moved to FileHistoryEntry in the io directory, and HistoryEntry made abstract.
As the abstract HistoryEntry cannot be instantiated directly, the ProjectManager is now responsible for creating new HistoryEntry.
git-svn-id: http://google-refine.googlecode.com/svn/trunk@973 7d457c2a-affb-35e4-300a-418c747d4874
