Feature Proposal: Include PATCH method

[dariok]

(this has also been mentioned in a comment to #22)

The PATCH method as defined in RFC5789 is especially useful in an XML database environment. I think it should be included in the available methods.

While in baseX allows the use of custom verbs in %rest:method, eXist does not – which means that PATCH requests are not possible in eXist.
With the PATCH method established in said RFC, I think it would be a good idea to include it so that there can be a standard way of RESTful patching.

[adamretter]

This has been added in #88 as %rest:PATCH