Redirecting to external content on GET of fcr:content

Resolves https://www.pivotaltracker.com/story/show/74627750
In order to test the desired use case, the extURL should point to a datastream within the federation.
A successful test demonstrates that a GET on the internal datastream resource returns the content found within the federation.
When I test this manually, a ModeShape exception is thrown.

```
javax.jcr.RepositoryException: org.modeshape.jcr.spi.federation.ConnectorException: The node key '87a0a8c7505d645cd4c5e7-0c2b-4783-b774-a07f17f5e600' does not belong to the 'fileSystem' source. Only nodes from that source are allowed. at org.modeshape.jcr.JcrSession.save(JcrSession.java:1184) 
```