[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 :)
`xlinkHref` is a thing as well --- I don't think we get much from `angular.isDefined`, it's more characters than just typing `!== void 0`