Theme passed down as context

[natew]

I think this is a great idea, referenced in this thread: https://github.com/callemall/material-ui/issues/30

We're already building a single theme object, the only difference would be to require the end-user to set it as a context. This would mean some docs explaining how to do that and a new release. So:

• [x] Set theme as context in example project
• [x] Change Styled to check context rather than using the function
• [x] Fix bugs, document, release

[natew]

Just documented this on http://reapp.io/docs-themes.html