piglit-summary.py: Don't allow both -s and -d options

The -s option overrides the -d option, so it makes no sense to allow
both options to be passed
piglit-summary.py: Convert to using new summary for generation

This changes the output to actually use the NewSummary class.
This produces no real change in speed, despite the fact that the new
method is compiled while the old method is not, most likely this is
overhead from the constructor for the NewSummary class