Remove unused gems; don't load assets group in CI.
Remove explicit load of diff-lcs.

It's a dependency of rspec-expectations.
Use unicorn instead of Thin webserver

This is in the name of eliminating unnecessary differences between
development and production. You must invoke unicorn with `unicorn`
rather than `rails s`. By default it listens on port 8080.
Remove watchr continuous-testing gem.

Nobody was using it :-(
Document what CanCan does in Gemfile.
Document the :assets group in Gemfile.

I think we can probably delete some of it (in particular compass-rails),
but don't know how to test it. Halp plz?
Document gems in test group
Upgrade debugger to 1.6.0

This should now include the headers for Ruby 1.9.3-p429 whose absence
was causing our builds to fail. Better late than never...
Merge pull request #240 from pozorvlak/gem_decruft

Remove some cruft from our Gemfile