Gemfile/gemspec cleanup

This cleans up a few things revolving around the gem setup:

- Removes Gemfile.lock and moves development dependencies from the Gemfile to the gemspec file. This approach is [recommend for gems](http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/).
- Add rake as a development dependency, since it's technically needed to run the Rakefile.
- I added .bundle and Gemfile.lock to the .gitignore. I also took the liberty to remove some of the other non-project files from gitignore, since those are often better suited for the global gitignore: https://help.github.com/articles/ignoring-files