connection: make pointer arithmetic ok for MSVC

Get pointer to containing struct; assumes uv_pipe_t embedded in a
ipc_client_ctx
connection: better container_of impl

Works with MSVC 8
test: add Catch unit test suite to build

Add reference to unit test suite and a simple unit test
to verify that all is working.
Merge branch 'add-catch-unit-tests' of https://github.com/smikes/Haywire into add-catch-unit-tests