Remove the extras directory

Previously, these contained support files for compiling MA27 and BLAS.
MA27 now has adequate build files.  We no longer recommend compiling BLAS.
Change the default locations of BLAS and MA27.
Update the distribution documents.

The documents were updated to reflect the deletion of extras/, new
recommendations for MA27 and BLAS.  Furthermore, we provided general
suggestions for compiling using Visual Studio, without stating that
support files exist.
Rebuild the HTML docs.