Adds lazy create context to getContext() method

Sometimes we need to work with the context object before a socket 
is opened e.g. to create poller object.
Merge pull request #145 from HeinrichHartmann/master

Adds lazy create context to getContext() method
Corrected ZMsg documentation.
Fixed recvFrame to return null on no data. Added Test cases.

Also added pretty printing to ZMsg object.
Merge pull request #149 from HeinrichHartmann/master

Fix: recvFrame() method of ZFrame does not return null if no data available
Apply checkstyle and sample changes
Merge pull request #150 from miniway/master

Apply checkstyle and sample changes
Extend support for ZMQ monitors to inline with jzmq
Merge pull request #151 from ealgell/monitorpullreq

Extend support for ZMQ monitors to inline with jzmq
Added double checked locking for shared variable context. getContext() and createSocket() should now be thread safe.
Merge pull request #153 from devinrsmith/master

Added double checked locking for shared variable context in ZContext.
Add unbind method to org.zeromq.ZMQ.Socket