Add is_proper_module for approximating gc roots

A 'proper' module is a module that's also in `sys.modules`. Only proper
modules should be considered gc roots. `show_chain` now continues until
it gets to a proper module.
Merge pull request #1 from fusionbox/is_root

show_chain shouldn't stop at modules that aren't in sys.modules

One example of such is Django's dynamically-created urlconf modules.