Added `ButtonGroup` and `ButtonToolbar` components

Added basic components for handling groups of buttons, group states
are added to the components via the bootstrap mixin.
Documented `ButtonGroup` and `ButtonToolbar` components

Documented possible states of this the button, uses the
http://getbootstrap.com/components/#btn-groups content as a basis for
this.
Removed bsVaration prop, removed example's `bsStyle="default"` prop

Added `justified` and `vertical` boolean props, see #35.
Removed unnecessary `bsStyle="default"` definitions. #36
Merge pull request #37 from pieterv/button-groups

Button group components and docs