Take 2: Fix issue #561 (bogus "Invalid Param name" error)

First attempt was both broken and ill-conceived.
Add Parameter::operator=
Removed Parameter::unregister_instance() in favor of a actor argument.
Adding Parameter comments.
One less comparison in the first gather of the merge sort
s/Param/Parameter/
Merge pull request #563 from halide/usercontext

Take 3: Fix issue #561 (bogus "Invalid Param name" error)
Merge pull request #564 from white-pony/min-max-optimization

One less comparison in the first gather of the merge sort
Bump minimum required cmake version (fixes #566)
Merge branch 'python3' of https://github.com/rodrigob/Halide into python3
Working on python3 support. Need to fix the div -> truediv issue.