Fix 'please install scripts to ~/.vim/bin' message, vundle compatibility

- Send the 'please install scripts...' warning to messages, so that it
can be viewed again with :messages
- Don't append vimbin to path before checking filereadable -- this was
causing it to fail to find the scripts in vundle/neobundle
- Move debug function to the top of the file, dump out runtimepath if
scripts not found and debug_flag is true