[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?
By which I mean, you need to change it to `require: '?ngModel'` instead of `require: 'ngModel'` --- otherwise if ngModel isn't there it will just throw