Added support for ztree

Ordered container class based on red-black tree.
Based on libredblack:
  http://libredblack.sourceforge.net/
Merge pull request #189 from shemminger/master

Add ztree class to CZMQ
Whitespace and formatting fixes
Merge pull request #190 from hintjens/master

Code style fixes
Fix sparse warning

Sparse style checker complains when 0 is used where NULL should be.
Make local test functions static

Callbacks only used in test code.
Merge pull request #191 from shemminger/master

Minor sparse related fixes
ztree: add missing autofree to ztree_dup()

Like zhash_dup, the newly created tree should have autofree.
ztree: fix comments around ztree_save/load

Correct some obvious copy/past oversights
ztree: add some tests for ztree_update

Need to check value overwrite path
ztree: switch to alternative source

Use a version of red-black tree with a more liberal license.
The main difference in this implementation is how the root node and
the nil node are encoded.
Merge pull request #192 from shemminger/master

ztree  update, fixes and redo with MIT code
Tentative fix for #187
Update flags to build on SmartOS / Solaris
zbeacon: improve search for interface to skip interfaces with null broadcast address, which happens on SmartOS / Solaris.

Refs #187
Merge pull request #193 from mattconnolly/master

Improved build support for Smart OS / Solaris
ztree.h was missing
Merge pull request #194 from hakankpunkt/master

ztree.h was missing in Makefile.am
libsodium not needed in Travic CI build
Updated man pages