Add Vue.js implementation

Vue.js is an MVVM library focused on the interface layer. It was created to meet the need for a nimble, lightweight interface layer during rapid prototyping at Google Creative Lab. AngularJS was too heavy and intrusive for that purpose; Backbone simply doesn't offer much help in the view layer. Ractive.js was a valid choice, but it wasn't composable and we do like Anguler's POJO models. The result is Vue.js, which is simpler, faster, uses POJO by default, and offers composable ViewModels. It is extremely efficient for rapid prototyping, and can also be competent for larger apps when used together with a module system (e.g. Component or Browserify). I believe it offers a valuable alternative to existing libraries by finding a sweetspot between functionality and simplicity.

Vue.js is currently being used for a few internal projects at Google Creative Lab. Unfortunately I cannot share these projects due to confidentiality :(

The project will be actively maintained and is thoroughly tested (96% coverage).

### Highlights

- Angular-style directives, filters, POJO models
- Backbone-style declarative, class-based API
- Reuse ViewModels as composable components (similar to React's composition)
- Expressive. Effective SLOC for this TodoMVC implementation: ~120
- CSS/JavaScript transition hooks
- Lightweight. 11.2kb gzipped.
- Blazing fast. see http://vuejs.org/perf/
- Ease of use. Drop-in usage or build with Component/Browserify.

### About this Pull Request

- [x] code styleguide observed
- [x] jshint pass
- [x] app spec observed
- [x] **browser tests pass (27/27)**
- [x] readme
- [x] learn.json

### Links:

- project website: http://vuejs.org/
- github repo: https://github.com/yyx990803/vue