Refine initialization of Vundle

- It added ~/.vim/bundle/vundle to 'runtimepath' before the has("win32).
  This was due to a merge gone bad.
- The call to vundle#rc() has been prefixed with 'silent!' and the
  :Bundle invocations are wrapped in if exists(":Bundle") to not produce
  errors if Vundle insn't installed.
- Wrapping the :Bundle commands gave them a reason to be indented :)