Added quit button in file menu
Added quick key to open and added exception catching around opening

Fixed an issue where if no repo was found in the the selected directory
or and sub directories then the program would crash. This was due to an
exception not being handled by the application. This is now handled and
a info message is now displayed if this occurs.
Changed the mainwindow icon to now be acGit.png
Added ability to quickly navigate to tags and branches

This was done by adding a new map which links commit location in the vector
with the oid of every commit for a quick lookup.