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

Replace ValueErrors containing implicit contract about the structure of the message with a RequirementParseError explicitly joining arguments for the string representation.

Instead of:

    $ python setup.py egg_info
    error in adminweb setup command: 'install_requires' must be a string or list of strings containing valid project/version requirement specifiers

We now have the more helpful:

    $ python setup.py egg_info
    error in adminweb setup command: 'install_requires' must be a string or list of strings containing valid project/version requirement specifiers.
    Error: Expected version spec in smsdk.authsvc>=0.0.8,2.0 at 2.0

It took me longer than it should to find the problem with the old error
message. The new error message would've helped greatly.

Check for Jython using sys.platform, not os.name