[WIP] Semantic-UI-Angular buttons

**WIP**: I have made initial design for buttons element. Currently I've created radio button directive, checkbox is coming. As Semantic-UI has "Elements", "Conditionals", "Modules", etc. I decided to group directives following that pattern (e.g. angular module for buttons look like 'semantic.ui.elements.button').

@caitp please take a look at it and I have a few questions: 
* Which prefix should we use ? I used "sem-" as you mentioned in 'old' repo.
* Semantic-UI has "Conditionals" buttons, should it be as a special case with its directive or we take it as normal radio button?
So it would mean that I need to create a directive for each sort of buttons in Semantic UI since we have changes on their styling/template? Basically, how do you see to use here a template when this concept is pretty dynamic and having a bunch of attributes does not seem so attractive? :)