Print message body in amqp[s]_listen[q] examples.

This fixes #199.
CMake: check for PKG_CONFIG_FOUND in FindPOPT.cmake
Stop CMake warning about @rpath on Mac OS X.
Use amqp_cstring_bytes() to specify locale field.
Add properties table merge function.
Add auth failure client capability as default.
Handle connection.close on auth failure.
Add CMocka as test dependency.