[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 :)
Hi @caitp, I know about transclude funtion. The complicated part of exactly "insert the nodes somehow" as you wrote.
I tried like this 
`transclude(scope, function(nodes) {
          element.replaceWith(template.append(nodes));
        });`
where template is anguar.element such as `angular.element('<a class="ui button"></a>')` but in this case I am loosing functionality of the watching attributes and custom css classes since element attrs are from directive.