Added responsive styles

[martinbean]

Added responsive styles, such as making the menu collapsible and content columns fluid and collapsible at small widths.

[zpao]

Whoa! First off, thanks for diving into this. And also for already signing the CLA :)

Secondly (and this is totally on us for not communicating it well): we actually have our site being built with Jekyll and keep the raw files in the master branch @ https://github.com/facebook/react/tree/master/docs (there's a readme there for getting set up in case you aren't familiar with Ruby/Jekyll). Since we build from that, it's important that changes to documentation are actually made there, and then we compile that and check it into the gh-pages branch.

[martinbean]

Hi Paul. No problem. I realised the docs were built with Jekyll, I just generated the site, and created a pull request of the generated files in the gh-pages branch and not modifications to the raw files in the master branch. So not an issue with communication but more a “D’oh!” moment from me.

How would you like me to proceed? Create a separate pull request containing my changes to the docs’ raw files in the master branch? Will you then generate/update the gh-pages branch from there?

[zpao]

Yea, a separate pull request against master is probably best. I don't think GH lets you repurpose a pull request against a different branch.

We'll then update gh-pages separately (so far we've rolled up multiple changes into one commit there)

[martinbean]

OK. New Pull Request will be coming in just a few moments!

[martinbean]

Updated pull request: #25.