Adds first test

Only one connect/accept and one send/receive operations
Implemented buffer management using the 'message_peek' flag

Also adds some tests.
Adds test
Fixes 'simple_exchange' test
Crux now prepends header to the data payload
Renamed variables and removed outdated comments.
Small bugfix (changed sizeof(0) to 0)
Multiplexer now knows nothing about receive_{input,output}_types
Merge branch 'MAID-466-add-headers'

Conflicts:
	test/core.cpp