removed unused source file (actor_facade.cpp)
added configureable result buffer size to spawn_cl

This is a new argument for the function spawn_cl. It defaults to zero,
which causes the implementation to calculate the buffer size according
to the number of create work-items.

The simple matrix example sets the buffer size manually.
added pfn_notify callback to print error messages

removed build log from program, since it is printed by the callback
Added asynchronous call to read memory from GPU.

Blocking calls result in undefined behavior.
added compiler options to program

+ unit test in test_opencl
Added test for variable result size

... and fixed bug in variable result size.