standard ruby impl

[jacobdanner]

I like the idea of your vim-bundle script, but had some issues with it on various platforms I was working on. I thought I'd give writing it using standard ruby libraries a try rather than shelling out. If you like these changes, the final change I'd make would be to git mv vim-bundle.rb vim-bundle, but thought keeping them separate may make the changes more readable.

I realize that you are probably working on a system with a proper shell (M$ command prompt with cygwin/rails instailler) so no worries if you aren't interested in this pull request. If not, I'd still appreciate any feedback on the changes as I'm still getting up to speed on Ruby and and sure I am missing many of the standard Ruby idioms.

Thanks, -jacobd