feat(checkbox): Implement radio-group and radio directives

These two directives allow for the creation of radio buttons with two-way data binding.

There are a couple of tests for these, but more are needed. It's likely possible to improve
this code a fair bit, as well.
fix(checkbox): Use angular.bind rather than Function.prototype.bind

Apparently TravisCI is getting an older version of PhantomJS without ES5 Function.prototype.bind(),
and this is causing tests to fail. Swapping for angular.bind.
Merge pull request #7 from caitp/radio

feat(checkbox): Implement radio-group and radio directives