Fixed some issues around the auto_import_cache

* fixes #361

* pymode_rope_autoimport_modules was never coming into effect during RopeContext __init__
* rope.regenerate now generates the cache for the entire project instead of just one file
* StringIO was been imported as a class, but been used as a module
Merge pull request #488 from brendanmaguire/develop

Fixed some issues around the auto_import_cache