Squirrel integration

- changed to .NET 4.5
- added nuget
- added nuspec file for releases
- Created basic events for UpdateManager
- Updater runs at the end of UIHasLoaded
- SparkleAbout now gets updates from the UpdateManager
- SparkleInviteOpener now manages protocolhandler installation and maintenance
- SparkleInviteOpener now handles github and sourcetree protocols, fixes #1570 for windows
- Discovered HKEY_CURRENT_USER\Software\Classes merges with HKEY_CLASSES_ROOT so no need to elevate privileges to add protocol handlers
- Close SparkleShare on Update or Uninstall
- Preparation for squirrel update