nmsg/base/encode.c: Add MSGPACK printing via libmsgpack.

Adds msgpack-c >= 0.5 dependency.
nmsg/base/encode.c: Add XML printing via libxml2.

Adds libxml2 >= 2.8 dependency.
Merge branch 'encode' of github.com:hstern/nmsg into encode

Conflicts:
	Makefile.am
	configure.ac
	nmsg/base/encode.c