improved compatibility: vundle should now support vim >= 7.0 (#2)

I've seen that vundle seemed to only run on vim >= 7.2 (mainly due to the reliance on fnameescape()), so I've decided to create the 'autoload/vundle/compat.vim' file to provide implementations for both "new" (and "supporting") vim versions, and also for those vim versions that didn't provide those functions.

I've tested using BundleList, BundleInstall, BundleSearch, manually getting the list of packages ('R' on the BundleList). It all seems to work fine (some of the plugins that you can load through 'Bundle' aren't really that good at detecting whether vim supports them or not).

These changes don't seem to have "worsened" vundle in any perceivable way (not at least in the way I've been using it), so I reckon you should be happy to pull :-)

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This includes changes triggered by jdevera's review comments.