LLVM/Clang support

this adds support for building with clang! (default on OSX. avail on Win and interesting alternative on Linux)

It's esp. nice for its debugging/analysis function. 

to build with clang, use: 

CC=clang CXX=clang++ ./build.sh 

tested with clang=3.2, clang=3.4

PS: we might want to reduce the build matrix, something like gcc-py27 & clang-py26? can we force that? 

Original [JIRA](https://issues.numenta.org/browse/NPC-280).