Allow to individually run CasperJS and QUnit tests

Allow to individually run CasperJS and QUnit tests and updated the task descriptions (useful when running `grunt --help`).

- `grunt casper` --> runs the CasperJS tests
- `grunt casper --path /path/to/test.js` --> runs a single CasperJS test.
- `grunt q` --> runs the QUnit tests
- `grunt test` -->  runs all tests

All commands also run JS and CSS lint tasks.