Add language highlighting option to notebook tag

Hey @jakevdp -- here's a random pull request for liquid tags! :sparkles: 

This adds an option to specify the language which Pygments should use for highlighting code cells.

    {% notebook filename.ipynb language[julia] %}

This may be helpful for those using [IJulia](https://github.com/JuliaLang/IJulia.jl) or notebooks in any other language, especially as the IPython project [broadens its scope](https://github.com/ipython/ipython/wiki/Roadmap:-IPython) of [language compatibility](http://jupyter.org/). By default, the language for highlighting will be ``ipython``.

This came up for me last night, hence the pull request.  It's working:

![selection_010](https://cloud.githubusercontent.com/assets/1284973/4076670/5bdcd63e-2ebc-11e4-99eb-9f6972b876d7.png)

Also:
- Updated `Readme.md` for this and the `cells` option
- Fixed a couple other small typos elsewhere in `Readme.md`
- Add tests for notebook tag regex

```
$ python -m liquid_tags.test_notebook
.......
----------------------------------------------------------------------
Ran 7 tests in 0.000s

OK
```