feat(editable): added editable directive

This directive is an implementation of editable modeled after the inline-editable version of http://vitalets.github.io/bootstrap-editable/. You can build and read the docs for how it is used.

Tests are in place, but if anyone finds a use case that I haven't tested against I am all ears! A couple of notes:

* There's an oddity in there at [#L207](https://github.com/hall5714/angular-ui-bootstrap/blob/aab6c80f3e393c07ebd18821478c9dce5f80a2b0/src/editable/editable.js#L209) of the source. The issue is that IE versions prior to 9 throw an error if an input type is changed. Unfortunately, Jasmine causes the error to be thrown. Frankly, I'm not sure if this will actually work in <IE9 or not, but this prevented jQuery from throwing the error. The error is specific to jQuery so running jQLite wouldn't throw the error anyway.
* Presently inputs attempt to fill the entire parent container in the templates. It may be better to pass in an inputClass.
* There is no popup support for now. This will have to wait until popups can have a template.

Any other questions feel free to ask!