Changed #defines to deal with older versions of gcc such that they don't
apply to code that includes zmqpp.hpp. This allows code compiled with clang 3.4
to include zmqpp.hpp without getting lots of errors.
Merge pull request #21 from michihennning/develop

Changed #defines to deal with older versions of gcc such that they don't
fix backlog option to be positive only, techically negative was accepted by zmq 2.x and 3.0 but didn't make sense
remove potential context leak from move operation, fixes #23
Fix potential leak in context, checked constructors to make sure all member variables are initialised. Actually fixes #23
rework of client to default to a more pipe friendly format, also fixes #22 (well improves rather than fixes really)
DSP-1228 related: improvement to error information
add zmqpp support for a socket 'label'
use string in preference to char*

request from Ben
fixed stupid error
create the include dir before installing to it
tweak for 3.3/4.0
fixed for merge issues