Improved documentation of zhash_pack/unpack

- added accurate grammar
- updated reference to FILEMQ RFC 35
Merge pull request #445 from evoskuil/master

Generate main project filters using GSL template.
Merge pull request #447 from hintjens/master

Improved documentation of zhash_pack/unpack
Problem: not closing messages on zmq_sendmsg errors

In some cases, was not properly checking for errors.
Updated NEWS for release 2.2.0
Merge pull request #448 from hintjens/master

Updates for 2.2.0
Updated version to 2.2.1
Removed references to deleted msvc projects
Merge pull request #449 from hintjens/master

Updated version to 2.2.1
Updated man pages
Merge pull request #450 from hintjens/master

Updated man pages
Added GSSAPI mechanism support
gss allow all
didn't mean to check this in
principle -> principal
props
Merge pull request #452 from connamara/gssapi

GSSAPI ZAP