Updated CMakeLists.txt so that CUDA_HOST_COMPILER is set to /usr/bin/gcc on OS X

This corrects the compiling errors experienced by users of cmake 2.8.10+ on OS X (reported in issue #30)
Type cast the parameter 'level' in wbLog() to type wbLogLevel

This corrects the Visual Studio compiling error noted on the course forum https://class.coursera.org/hetero-2012-001/forum/thread?thread_id=1521&post_id=5702