Adding custom source paths to rope scans

Currently SPI (this plugin) resolves imports only relative to the root directory of the open project. This means that if the top directory in the project other than the top module directory, absolute imports (e.g ```import project.foo.bar```) are not resolved correctly.

To overcome this limitation, I suggest to add a feature that enables the user to add custom source paths in the project settings, similarly to how it was done in SublimeRope .ropeproject file. This path should be added either to the python path or perhaps be used in the rope lookup itself.

Not sure which is the best way to solve it, but I'd like to give it a try on the weekend. Does anyone has a preferable method to solve this?