Added support for checkbox 'X'

The default checkbox style is a check/tick (http://ionicframework.com/docs/components/#checkbox).  These changes allow it to be changed to a cross 'X' with the addition of a class:

~~~
<li class="item item-checkbox">
     <label class="checkbox cross">
       <input type="checkbox">
     </label>
     Flux Capacitor
</li>
~~~

![ionic-checkbox-cross](https://cloud.githubusercontent.com/assets/842078/2559375/c3fabdc8-b778-11e3-884c-f671290ff910.png)

If no extra class is applied, it is set as default.  Or the `check` class can be applied.

Would update the docs, but need to do some more research on how to do that properly.

I'm not sure if the `:not(.cross)` idea is the best to be honest, as it'll need to be updated if anymore styles are added, but it works for this purpose.