Allow linking to locations within grains

Added a query parameter to the grain route named `location`, ie: http://local.sandstorm.io:30000/grain/Jn7kgDQy3XLfBkfLRoj5g8?location=/ghost/settings. Locations must be relative, and this is enforced in proxy.js.

You can set the location from within a grain iframe with:
```
parent.postMessage({'location': '/ghost/settings'},
  location.protocol + '//' + location.host.slice(location.host.indexOf('.')+1))
```
Hmm, I wonder if we want to use `replaceState()` rather than `pushState()` here. We don't want to extend the browser's "back" history because the iframe already has its own history.

I actually don't know how the iframe's history and the shell's history interact, so you might want to test a bit to find out.