Squirrel integration

- changed to .NET 4.5
- added nuget
- added nuspec file for releases
- ignored nuget related files
- added build process for squirrel packages

Now with all the benefits of [Squirrel.Windows](https://github.com/Squirrel/Squirrel.Windows)

ToDo:
[ ] - Manage user data between releases #Squirrel/Squirrel.Windows/issues/47
[ ] - Provide install/upgrade path for windows protocol handlers
[ ] - Manage other installation related events (onInitialInstall, onAppUpdate, onAppUninstall, onFirstRun)