pipeline/processor API: use URI not String for url references

> The org.orbeon.oxf.pipeline.api.PipelineDefinition class has a couple of
> methods to set inputs:
>
> void  addInput(String name, Element element)
>           Add an input with the given name and dom4j Element.
>  void addInput(String name, String url)
>           Add an input with the given name and URL.
>
> The addInput(String,String) method should really use a java.net.URI to
> pass in the url reference rather than a String (not java.net.URL: URI is
> just a string in a certain format [RFC 2396], whereas URL is a 'live'
> resolved location, doing nasty things like hitting the DNS server &
> trying to find stream handlers).
>
> (a) this would make it much clearer that the String input shouldn't
> contain an unparsed xml document string (which is what you assume unless
> you look at the javadoc, because the other method takes a parsed xml
> element), plus (b) it means badly formed url references can't be provided.
>
> There are a bunch of places downstream which stick to using Strings
> instead of URIs: these are more 'internal' rather than API classes though:
>     PipelineUtils.createURLGenerator()
>     URLGenerator
>     URLFactory
>
> Aside from making the API a bit clearer, using a URI means instead of
> trickery like (URLFactory):
>
>     spec.startsWith(OXFHandler.PROTOCOL + ":")
>
> you can just do
>
>     spec.getScheme().equals(OXFHandler.PROTOCOL)
>
> Adrian