[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, one of the problems with radio buttons + ngModel is that, we end up with an instance of `ngModel` for each radio button, and it makes things more complicated than it really should be. It's one of those annoying things that we'd really like to fix, but unfortunately it is one thing that can sorta "go wrong".