Add better logic for loading entry points

As noted in the code, setuptools 11.3 deprecated EntryPoint.load and
its require parameter. Starting with 11.3 the preferred way is use
EntryPoint.require and EntryPoint.resolve as necessary. Unfortunately,
those methods do not exist in all versions of setuptools. So we have
to check for them and use them when they're available. Otherwise, we
fallback to using EntryPoint.load with the require parameter.

Closes #59
Closes #90