extract JasmineRails::Runner to support alternative non-rake test runner...

I'm working on adding a non-rake test runner to my app, and this seems like a good opportunity to cleanup the code a bit and refactor the rake task into an actual `JasmineRails::Runner` module.

**NOTE:** this is intended to be *strictly* a refactoring and no functionality changes are intended as part of this re-organization.

any feedback is welcome on the approach!