Add util methods
 - get_path_info() - simply returns the PATH_INFO if available
 - get_var() - get var from an array - checks if index is set & can validate type.
Use util method - get_path_info()
Auto advance the signature HEAD (and migrater) as patches get applied.
Add the ability to set persistent upgrade mode ajax or manual
Add ability to switch to manual upgrade mode on ajax failure - 404 error.
Merge pull request #500 from protich/feature/upgrader_revisited

Upgrader revisited