Merge pull request #143 from miniway/master

Fix wrong Router xwrite_activated assert
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