feat(focusable): allow control to be get focus. Support for tab navigati...

Changes:
* Accesible for tab-key navigation
* Can start typing even when the dropdown isn't open, just with focus
* Can open dropdown with enter, up-down-left-right keys when focus
* Isn't focusable when the control is disabled

Working [plunker](http://plnkr.co/edit/DWpNaLT0AdsdRBvELynD?p=preview)

Resolves #51