Add support for arbitrary function namespaces

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Currently Functions do have to use the the

  fn: http://www.newmedialab.at/lmf/functions/1.0/

namespace.

This is fine for the built-in functions and still ok for general purpose 
function. However for domain specific functions it would be more intuitive to 
not restrict method names to a single namespace.

The proposal is to allow the use of any URI token as function name.

This would require to update

* the LDPath parser to support uris
* the LDPath configuration to accept function that use URIs with a namespace 
different to 'fn'

To add additional functions for specific domains it would be more in

Original issue reported on code.google.com by [email protected] on 13 Mar 2012 at 8:21