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))
```
As discussed, let's have the path appear after the grain ID, e.g. if you went to:

    http://localhost:2080/grain/7fwBstbqw443XwsoRQv9zd/foo/bar?baz=qux

Then the location is "/foo/bar?baz=qux". Don't forget to url-escape this when passing as a query param to _sandstorm-init.

Also let's generally rename "location" to "path".