- rename s:check_bundle_name to s:register_bundle_name,
  it has the side-effect of updating the bundle name registry
- expand possible return values
- re-registering a plugin name for the same bundle is no conflict
- don't continue processing plugins if registration fails

- ensure that all system calls go via installer.vim
  (vundle#installer#system, used to be s:system)
- escape/wrap complex command sequences
- log commands before trying to execute them

- only test for os, vim version and patch level once
- on old vims, only wrap system argument for cmd.exe