Replace atlas with multithreaded OpenBLAS to speed-up on multi-core CPU for the boost-eigen branch

Multi-threaded OpenBLAS makes a huge performance difference. The benchmarks with and without it in comments to #16 demonstrated more than 5 times speed-up for boost-eigen and MKL on a machine with 4 Hyper-Threading CPU cores (supporting 8 threads).

This fixes #79.