Add an LDPath function to retrieve a back-end defined response header

With the `fn:header` function, LDPath implementations can retrieve "headers" from the backend (which may be HTTP headers for some backends, but may be other metadata about the resource for others).

e.g. 

```
content_type = fn:header("Content-Type") :: xsd:string ;
```

Could result in:

```
content_type = {application/rdf+xml}
```