Removed guard in favor of grunt-contrib-watch, also improved watch tasks

This removes all gem dependencies.

`grunt watch:scripts` watches for source changes and rebuilds dc.js, etc.
`grunt watch:tests` runs the tests whenever test files are modified.
`grunt watch:jasmine_runner` updates the jasmine_runner.html file whenever jasmine tests are modified.
`grunt watch:reload` should be equivalent to guard livereload.