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))
```
Bug. :)

As discussed, we know exactly what the origin should be, so let's compare against that.

There's no guarantee that the wildcard URL relates to the base URL (in fact on demo and alpha your check here wouldn't work).