Add HTTP PATCH Method

Quoting the abstract of RFC 5789:
	Several applications extending the Hypertext Transfer Protocol (HTTP)
	require a feature to do partial resource modification.  The existing
	HTTP PUT method only allows a complete replacement of a document.
	This proposal adds a new HTTP method, PATCH, to modify an existing
	HTTP resource.
https://tools.ietf.org/html/rfc5789
Merge pull request #22 from tiwoc/http-patch

Add HTTP PATCH Method.
Thanks to Daniel Seither for the pull request.