Add a Watch Option

This adds a new option, `watch`, that is a boolean that will run `compass watch` instead of `compass compile`. This option provides a solution to both #14 as Compass's native watch task is significantly more performant than calling `compass compile` every time you would like to compile your Sass, as well as a solution for #77 as the output from Compass Watch is now streamed to the console and will show errors.

In addition, I've updated the README to describe the new option and warn how using it turns this into a blocking task, and suggests other Grunt projects that will be able to assist in dealing with blocking tasks.