Improve shell escaping
Remove debugging statement
Fix backgrounding
Really support Rails 4 commands

We can no longer use ruby -C, so we have to chdir inside of Vim instead.
This has the nice side effect of better supporting ruby version
managers.
Fix rails#app().has('bundler') on Bundler 1.3
Fix :Rrunner
Add Mongoid gf support

Closes #175.
Remove :Rmetal
Remove :Robserver

They're gone from Rails 4, and easy enough to access with :Rmodel at any
rate.
Bring default generators in line with Rails 4
Remove plugin support

Left behind are some path changes, since those are trivial to support,
and :Rplugin, so it continue to deliver its referral to :Rlib for the
Gemfile.
Support extracting concerns and helpers

Closes #28.