Corrected issues when compiling on Linux

[LucasCampos]

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 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