Add a new site-search component

This is basically a search paper-icon-button + a paper-input that fires off to a [site:] Google search for a given domain (by default, the current domain).

I've also added a "condensed" mode, where we only display the search button, and clicking it expands and shows the input field.

![search](https://cloud.githubusercontent.com/assets/1369170/5146994/bf4718b4-7166-11e4-9bb5-1bb74b66f961.png)


Hmm, add a `devDependency` on `polymer-gestures` to be safe (it's only a dependency of dev-mode polymer)