[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 :)
> provide additional css class Button how to copy it to the directive's template ?

If it's a `replace: true` template, then attributes from the calling template are added to the replaced template. This means that you can't use `ng-class` in the template's top-level element, though. (or `id`, or anything that is an angular expression)