Document Pow support
Basic :Rpreview subdomain support

Use :Rpreview admin to go to admin.lvh.me:3000.
Include "ruby" in commands only on Windows
Detect any first level directory

This was a premature optimization from before I knew any better.
Drop support for config/rails.vim
Remove :Rplugin
Remove deprecation warning from :Rinvert

This was just a feeler.  Now that I know it's going, there's no need to
continue reminding people.
Support literal filenames in projections