Add possibility to use OpenBlas
Merge pull request #325 from AlOa/OpenBlas

Add possibility to use OpenBlas
rename python include config var to match lib
Give choice of ATLAS, MKL, and OpenBLAS (with option to override paths)

- configure build for ATLAS, MKL, or OpenBLAS on Linux and OSX
- allow overriding of the include or lib dirs
- replace magic numbers with BLAS names (atlas, mkl, open)

Follow-up from #305 and #325.