Add Vue.js implementation

Vue.js is an MVVM library focused on the interface layer.

### Highlights

- Angular-style directives, filters, POJO models, but much lighter and less intrusive;
- Backbone-style declarative, class-based API, but much more powerful in the view layer.
- Reuse ViewModels as composable components (similar to React's composition)
- Expressive. Effective SLOC for this TodoMVC implementation: ~120
- CSS/JavaScript transition hooks
- Lightweight. 10.7kb gzipped.
- Blazing fast. see http://vuejs.org/perf/
- Ease of use. Drop-in usage or build with Component/Browserify.

Use case wise, it is very similar to Ractive.js, but offers quite a few advantages: Vue.js is lighter, faster, composable (a pain point for Ractive) and uses POJO by default (by supporting IE9 and above only). It is extremely efficient for rapid prototyping, and can also be competent for larger apps when combined with a module system (e.g. Component). I believe it offers a valuable alternative to existing libraries by finding a sweetspot between functionality and simplicity.

I'm the author of Vue.js and am currently using it in a few internal prototypes at Google Creative Lab. Unfortunately I cannot share these projects due to confidentiality :(

### 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