Allow relative URLs in annotations-by-document request

[spyysalo]

Annotations can now be requested by document using the syntax

http://example.org/documents/{doc-url}/annotations

As documents don't need to be local, the implementation uses absolute document URLs, as in e.g.

http://example.org/documents/http://example.org/documents/123/annotations

However, these are ugly and unwieldy. The relative variant

http://example.org/documents/123/annotations

should be supported as an alternative.