Import/upgrade basic grunt tools into 3.x skeleton.

[beporter]

Old README.md.template block:

### CSS Changes

* CSS is managed via LESS source files.
* LESS source files are located in `webroot/less/`.
* Run `grunt less` to process the `webroot/less/` files and output compiled CSS files in `webroot/css/`.
* **Commit both the .less and the .css changes** back to the repo as you work.
    * (Until a CDN is set up, static assets will be served from the app server directly.)

.less files of note:

* `global.less` is included everywhere in the site.
* `public.less` is referenced in only the default (public) layout and will override anything in global.
* `admin.less` is referenced only in the admin layout and will also override global.
* LESS resources can be broken out into logical files and `@import`ed into global, public or admin collections as appropriate.
* By default, bootstrap is `<link>`ed separately in the default.ctp layout, but can be rolled into global.less if desired.


### Other grunt Commands

* _(no args)_ or `watch` - Starts the file watcher and auto-executes tests for any php file that changes (and has a test file associated with it.) Also watches .less files and compiles them into CSS on change.
* `less` - Compiles .less files into CSS.
* `test` - Executes the Mocha Javascript test suite.

These were good basic tools to have available in all projects and I lament their loss in the current Cake 3 skeleton.

The run-tests-on-save command in particular is going to need some new logic written for it, since we're engaging PHPUnit directly in Cake 3 projects instead of going through the Cake TestShell that was available in 2 but that's not insurmountable since the src/ and tests/ directories still mirror each other nicely.

[beporter]

Thanks to @kriskd, these tools are pretty well fleshed out in the EU rebuild. We should use that as a starting point.

[beporter]

The set of commands isn't exactly the same, but basic grunt tooling will be available soon. The README.template needs to have it's CSS and JS sections updated to account for this still.

[beporter]

Notes added to README. Safe to close this when #168 is merged.