Uris for repos specified as user/repo end up being https://github.com/user/repo. For some reason, github allows cloning from such uris, however, it is better to rely on the addresses that github provide for cloning, and those include the trailing .git

This change appends .git to github repo uris when needed.

- otherwise it defautls to vim-scripts repos
- closes #69
- TODO: pass options aswell

- closes #37
- join is useless there

GitHub usernames cannot start with an underscore, but they can be a
single character long.

- ie, like this
Bundle 'gmarik/vundle'

- this was intended for case when user uncomments installed `Bundle` and
reloads .vimrc; which should make the bundle available
- TODO: later

- fixes https://github.com/gmarik/vundle/issues#issue/3
- TODO: not the most efficient approach

- so it's ready to use right after installation

- vundle.vim - entrypoing and loader
- config.vim - configuration and runtimepath management
- installer.vim - installation and docs