Add support for using openblas for the _dotblas function.
BLD: Update with information about openblas.
Make vecLib / Accelerate not shadow other optimized builds under OSX
site.cfg.example should not reference blas_opt and lapack_opt

blas_opt and lapack_opt sections are not actually read by the
numpy.distutils.system_info machinery. They are just meta entry points for the
`get_info` public function that in turns introspec the atlas, openblas and mkl
sections.
Use the default install location of OpenBLAS in the example
Merge pull request #3642 from jseabold/openblas-install

BLD: Allow easy use of openblas.
DOC: Update release notes for openblas build.
Merge pull request #3688 from jseabold/update-release-notes

DOC: Update release notes for openblas build.