Improvements to the installer

* Checks if the installer is being run as administrator and aborts (we
cannot allow this yet as it would prevents updates from installing; I
might fix this in the future, but it's not easy)
* The app is specifically launched at user-level on finish even if the
installer is running as admin(which would cause problems). Even though
the installer is prevented from running as admin at the moment, it might
be possible in the future
* The selected installation directory is checked to see if it's
writable, if not installation does not continue (this prevents the user
installing to Program Files)

With thanks to the UAC plugin for much of this functionality:
http://nsis.sourceforge.net/UAC_plug-in