Remove requirement that element must have unique ID and add binding for data and renderer

Hi,

I have modified the directive to 
- remove requirement that element must have unique ID 
- add binding for data and renderer as attribute `vega-data` and `vega-renderer`. This way the spec can be reused with different data. Also, the directive now watch for changes in the data and update the vis dynamically.

Thanks,

Krist

[screenshot](https://cloud.githubusercontent.com/assets/1659771/2750025/9eb807a2-c842-11e3-8510-26dd11a33d54.png)