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

This is a work-in-progress to improve installation.

- [x] document BLAS choices
- [x] include commands for ATLAS installation
- [ ] detect platform in Makefile by `uname` 
- [ ] 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)