Dev dependencies

Hi,

These commits add `rspec` as a development dependency, allowing other devs who wish to hack on this gem to issue a common-or-garden `bundle install` to install the correct (I think) version of rspec to run the specs. It also adds `Gemfile.lock` to `.gitignore` as [encouraged by Yehuda Katz](http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/).