Remove the 5 'exec' calls definining polynomial classes.

Currently there are 5 places under numpy/polynomial which do
something like:

  exec polytemplate.substitute(name='Hermite', nick='herm', domain='[-1,1]')

I have changed those so there are start/end markers, like

  ...

I have edited  polytemplate.py so that it implements a __main__
which finds those block markers and replaces the content with
the appropriate "polytemplate" substitution.

This means that if you edit the template then you will need
to manually run 'polytemplate.py' so that the dependent files
are rebuilt based on the new template.

The performance of

  python -c 'import time; t1=time.time(); import numpy; print time.time()-t1'

goes from 0.079 seconds to 0.057 seconds (best of 10), for
an import speedup of ~25%.