Make it easier to return files (LiftScreen?)

[hedefalk]

Discussion thread: https://groups.google.com/forum/#!searchin/liftweb/return$20file/liftweb/uz6EaJ3fWEg/-F5upfvJZM0J

[github-importer]

Imported from Assembla: http://www.assembla.com/spaces/liftweb/tickets/1108

[Shadowfiend]

@dpp saw you weren't exactly clear on how you envisioned this working. Any hints to add there, or should we just lean on ResponseShortcutException? Or perhaps add a well-named helper to S much like redirectTo that is for generic “return this response instead of whatever's happening right now”, and just throws the appropriate ResponseShortcutException…