Add new functions to zframe.txt and zmsg.txt

As requested by @felipecruz in https://github.com/zeromq/czmq/pull/158
this pull request adds the new functions to the documentation.

As zframe_sendmem will probably end up in zsocket, I didn't add that yet.

Isn't there any tool to auto-generate the docs from the header files? The function docs seem to be that same as the header files.