Document BLAS install, tailor Makefile and Makefile.config to platform

This documents BLAS choices, fixes the ATLAS BLAS / non-MKL build configuration, and otherwise eases installation by automatically configuring the build according to the platform.

- [x] document BLAS choices (ATLAS and MKL supported, OpenBLAS for the adventurous)
- [x] include commands for ATLAS installation
- [x] detect platform in Makefile by `uname` 
- [x] correct cblas/atlas linking on OS X (needs `-framework vecLib` and other details)
- [ ] ~~possibly split Makefile.config into Makefile.config.linux and Makefile.config.osx (as once suggested by @sguada)~~ No need, given that the `Makefile` now makes the proper choices.