Added zbase64 documentation.
Added gsl script to generate new class.
Renamed zbase64 to zarmour.
Moved zclass packaging to non-generated files.
Fix compilation error

This issue was introduced with PR #747, which has forgotten to also add
list 'src' part of SUBDIRS.

Closes zeromq/czmq#751
Corrected some inaccuracies in zclass_example.xml documentation.
Updated zarmour.xml to reference correct script name.
Fixed error for zclock_usecs prototype