LLVM/Clang support

this will add 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. 

We are just one step from the goal :) I commented the << logging and rest compiles, a few tests fail due to this logging hack, rest looks good. 

Please have a look. 

PS: we might want to reduce the build matrix, something like gcc-py27 & clang-py26? can we force that?