Change head to svn server

The git server was just a mirror of the svn server. This is the actual
lammps repository.
Add revision 9253 support
Add direct download from 2013-01-04_17:50
Make 'all-standard' the default installation, no longer an option
No longer depend on 'scons' but use 'env :std' instead
remove all-standard option which was obsolete
Cleanup comments and caveats
Remove trailing whitespace
Add mpi example to caveats