Add RSpec setup with rspec-rails

Generated using `rails generate rspec:install`, with three small changes:


#### Remove --warnings flag from .rspec file

This causes warnings to be printed about all of your dependencies, which is untenable for a Rails project.

See http://stackoverflow.com/a/24177563.


#### Disable profiling of 10 slowest specs

This can be useful if you have a large, slow test suite, but until then it's just noise.


#### Remove special treatment of specs tagged :focus

This adds noise to the output on every run, and we don't use it.