Add Eco syntax highlighting

I couldn't find any TM bundle with support for @sstephenson's wonderful [Eco](https://github.com/sstephenson/eco), so I hacked together my own (based on ERB)

It's far from perfect (for one thing, the highlighting screws up a bit when in an HTML attribute), but it's much better than nothing.

It looks like this:
![screen shot 2013-11-08 at 23 39 51](https://f.cloud.github.com/assets/183747/1504965/27188508-48c9-11e3-940c-87deead6865d.png)

Any Eco fans out there who could help make it better? :)