[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 :)
it's not realistically useful for that.

1) this information is already available via developer tools (such as the accessibility tree)
2) in cases where it makes sense (eg anywhere this function would be used), we can intuit the aria-label to use if one is not provided.