Big performance fix for find_packages by ignoring hidden dirs earlier

Upgrade packaging to 15.3

Don't quote the executable name in JythonCommandSpec.from_environment() since it is quoted if necessary in CommandSpec._render().

With the executable quoted on initialisation of JythonCommandSpec, the quotes get escaped in the resulting header, eg:

#!/usr/bin/env \"/path/to/jython\"

Detect Cython later in the build process, allowing the library to be specified in setup_requires. Fixes #288.

Backed out unintended changes made in f572ec9, restoring use of 'imp' module for dynamic module creation originally committed in 06ac3674 and 4c121bd24f.

Expand the range of valid operators to include comparators