zpipes server implements zpipes protocol

- detects end of pipe, when writer closes pipe
- client requests can be pipelined (sent in advance of replies)
- implements timeout on fetch requests
- selftest now covers timeouts and end-of-pipe conditions
Removed old zpipes classes

- zpipes_server replaces zpipes_agent and zpipes classes
- zpipes_client updated for new protocol
- fixed API exports in zpipes.h
Fixed #10
Fixed issue #11

- renamed overall packaging to "zbroker"
- still using zpipes_xxx for zpipes-specific classes; this may need to
  be changed later.
README and other small fixes
Fixed build errors

- was referring to old zpipes.h
- some security warnings for zsocket_bind calls
Merge pull request #9 from hintjens/master

zpipes server implements zpipes protocol