feat(uiStateActive): directive to add/remove classes for active state

Fixes #19

**TODO:** Still needs a test suite, I'll try to write some stuff tomorrow. In the mean time, [here's a prototype](http://plnkr.co/edit/7EBvfxLRsgtjgQfDA5ff?p=preview).

---

Adds a directive to switch a set of classes in a DRY fashion by interacting  with neighboring ui-sref directives.

The class to be used is an interpolated string, which is not $observed. There is no default, as I do not wish to tie this module to a particular CSS model or framework. A default may be configured in the future with a provider. As usual, multiple classes may be used, as Attributes.$addClass/$removeClass will separate classes on whitespace.

**CAVEATS:**

This is only capable of working as a parent of a ui-sref directive, or as a sibling directive on the same element. In the future, there may be a mechanism around this, but for the time being I can't think of what it would look like.

**USAGE:**

Sibling directive on same element as ui-sref:
```html
<a ui-sref="foo.bar({baz: '123'})" ui-state-active="active" href>123</a>
```

--- OR ---

Ancestor directive of ui-sref:
```html
<li class="item" ui-state-active="active">
 <a ui-sref="foo.bar({baz: '123'})" title="123" href>123</a>
</li>
```