Merge pull request #448 from hintjens/master

Updates for 2.2.0
Merge pull request #449 from hintjens/master

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

Updated man pages
Added initial implementation of zsock.
Added zsock_t specific sockopt generator
Updated the project.xml and the (generated) makefiles
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
Added zmsg and zframe send and recv methods to zsock. Fixed selftest.
Merge pull request #453 from mditzel/master

Added zsock_t class (see issue #451)
Fixed the Travis build 'redefinition of typedef' warnings (= errors).
Merge pull request #454 from mditzel/master

Fixed the Travis build 'redefinition of typedef' warnings (= errors).
added doc for new zsock class
Merge pull request #455 from sappo/master

Added doc txt for new zsock class
Distribute zsocket_option.h in library
Merge pull request #456 from sappo/master

Distribute zsocket_option.h to library