Implement JDS copy_script method. Refactor to type-codes. Other fixes.

While broken, implemented JDS.copy_script. (It includes HTML form
boundaries in script file upon posting.)

Adds type-codes for file types, and uses global lists for collections of
file types.

DistributionPoints.copy(_pkg)(_script) now have defaulted id_ parameter
to work with JDS' copy methods.

Add documentation for above.

Fix reference to jss object in JDS.exists()

Implement JDS.exists_with_casper(). Algorithm is unsatisfactory, so
expect it to change.