Grunt

First stab at making a Grunt task runner for Timber.
- `grunt` default task watches for changes in `/stylesheets` and concatenates them into a master `timber.scss.liquid` file, then uploads the master via the theme gem
  - Order is determined by `@import` statements in `stylesheets/timber.scss.liquid`
- Also has `build` and `zip` tasks
  - `build` concats css, and optimizes images
  - `zip` does the above, but also creates a zip file with only the desired files for uploading the theme

To do:
- Could use some stuff from https://github.com/graygilmore/timber-grunt
  - Could make sense to use https://github.com/wilr/grunt-shopify 

cc/ @stevebosworth @mpiotrowicz @graygilmore