Add tests capturing expected behavior, including failure to match expectation indicated in docstring.

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.