Only install completions for the "drush" command

This is a potential fix for issue #915. I've tried to document the procedure everywhere that aliases/bash-completion are already mentioned in the docs.

Would it be possible to not to modify drush.complete.sh since that is a Drush file and we usually don't modify those? 

Here is what I typically add to my .bashrc:

```
    if [ -f `which drush`/../examples/example.bashrc ]; then
     . `which drush`/../examples/example.bashrc
    fi
```
Could I add another line that added completion for dr? Something like `complete -o bashdefault -o default -o nospace -F _drush_completion dr`. Would I then get completion for both dr and drush? I'd rather ask people to modify their bashrc to handle additional aliases.