Corrected issues when compiling on Linux

There were two problems when compiling aicore

i) NULL not defined
ii) Programs were not being compiled with -lrt.

The first issue was solved by adding #include <cstddef> to aicore/include/aicore/aicore.h.
The second issed was solved by adding 

IF(UNIX)
        target_link_libraries(aicore -lrt)
ENDIF(UNIX)

to aicore/build/CMakeLists.txt.

These errors were found and solved using Ubuntu 12.04, g++ 4.6.3 and cmake 2.8.7