Merge pull request #855 from hintjens/master

Man page fixes
add proxy_chain, a multi proxies chaining in the same thread feature
Merge pull request #858 from lalebarde/master

add proxy_chain, a multi proxies chaining in the same thread feature
Fixed man page for zmq_proxy_hook

- spaces instead of tabs for indentation
- fixed levels under EXAMPLE USAGE
- added to Makefile.am