Move Projectile's menu item under Tools.
Introduce support for finding tests and switching between code and tests.

`projectile-find-test-file` gives you a quick way to navigate to test files
within the project. `projectile-toggle-between-implemenation-and-test` gives
you the ability to toggle between the code and it's test file. Currently this
works only for Rails projects using rspec, but it will be extended to support
other popular project types.
Remove projectile-open.