Create a framework for managing user interface components in iframes

This is the result of [this conversation](https://github.com/philc/vimium/pull/1382#issuecomment-68214559) between @smblott-github and I on #1382. It creates `UIComponent` in the frontend and `UIComponentServer` (provided by `pages/ui_component_server.coffee`) for the iframe(s).

This also includes an example (provided by commit d20caa49075c0605ec2196416a5633d5e205b615). The example
* opens/closes with the help dialog.
* refreshes its contents every time it is opened, showing
  - the current vimium version (as requested by Stephen), and
  - a random number (to confirm that the contents have been refreshed on each reveal).

@smblott-github, can you take a look over this?
Why do we need these two methods at all?  I'd like to see them go.  The caller can tell the framework what styles it wants in the constructor.  Then we're done.