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))
```
The recommended way to implement "startsWith" is:

    location.lastIndexOf("/", 0) === 0

This avoids searching the whole string.

Comment that we need to refuse a leading "//" because the browser will interpret the resulting `Location` header as directing to a different domain.