[WIP] Semantic-UI-Angular:  sm-button  directive

**WIP**:  sm-button is a generic button directive. I took as a base idea the design from angular-material.

A few thing to note:
* I have created a service (ariaHelper) to check if an element has aria specific attribute.
* Currently I've decided to use "sm-" prefix. I liked it more than "sem-".
* Unit tests are coming.

@caitp reconsidered design and second attempt :)
When `transclude` is set to `true` or `'element'`, you get a transclude function passed to your link function and controller.

```js
link: function(scope, element, attrs, controllers, transclude) {
  transclude(scope, function(nodes) {
    // insert the nodes somehow
  });
}
```