Implemented full protocol message filtering

- Engine can now process PINGs while waiting for a READ to finish.

- Added extensive test cases to zpipes_server.c.

- Fixed #30 - can now close pipe while waiting for READ to finish.

- Fixed #26 - idle clients are disconnected (defaults to 60 seconds).
Updated grammar in XML and README.md
Added syslog support & configuration

- uses new CZMQ zlog class
- zbroker.cfg shows configuration options
- server/background to run in background
- else logging is copied to foreground as well as syslog

Right now we log the server start/end; we will add futher logging as needed
over time. Note that animation and tracing are still sent to the console,
as these are really for debugging and should never be used on production
code.
Brokers can now detect each other

- on same machine, or same LAN
- using zyre to detect and interconnect

Still to do:

- Make it easier to specify the IPC name for a broker; currently you
  have to create a .cfg file per broker instance.

- Build pipe read/write protocol on top, so readers and writers can
  find each other across the network.
Merge pull request #33 from hintjens/master

Implemented full protocol message filtering