[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?
But in case we would use this directive as an element it has to have its template? That's why I only consider a use case as an attribute.