adding codemirror for playground

This adds [CodeMirror](http://codemirror.net/) (from [cdnjs](http://cdnjs.com/)) for the editor section of the playground (document, frame and context). Here is my [gh-pages](http://bollwyvl.github.io/json-ld.org/codemirror-playground/playground/): 

Certainly open to including the CM source directly or as a submodule, but as this repo gets checked out in tests, I figured I'd err on the site of repo thrift.

I saw this mentioned a few places re: GSoC, but hadn't seen any PRs out there that had done it.

It uses [jsonlint](https://github.com/zaach/jsonlint/) which highlights where problems occur in the editors, which is basically what I needed while dinking around with examples.

The existing playground is already a useful tool. I look forward to improvements here, especially in conjunction with #291, and I hope I can be of additional help!

It's probably good to go as is, but I am interested in feedback, both on the current implementation as well as on:

### Easy(ish) next steps
 - better highlighting: figure out how to reuse the prettify language definition to make an enhanced codemirror mode. at that point, CM could be used for input and output, which would reduce page load
 - autocomplete hints: the `@` items could be done very easily. prefix introspection may be possible as well
 - prettier reporting of JSON-LD errors

### Refactoring
 - jQuery UI is probably not necessary, as bootstrap can already do [tabs](http://getbootstrap.com/2.3.2/javascript.html#tabs)

### Future integration things
 - [macros](https://github.com/antoniogarrote/json-ld-macros)?
 - [vega](https://github.com/trifacta/vega)?
 - [bl.ocks](http://bl.ocks.org)/[gists](https://gist.github.com/)?