Better relative paths handling and returning more useful values from API

I'm not sure that a suggested fallback from incubate to surround will work on windows, but looks logically.

Returning path lists, that were just loaded, is very useful for plugins, that use pathogen's internal API. i.e. I've created my own vim-profiles plugin, that uses this feature. Also vim-ipi may benefit from this change, since now it is forced to make extra rtp value comparison before and after pathogen invocation to detect loaded plugins.