Handle ':bd' correctly, do not quit Vim

Fix techlivezheng/vim-plugin-minibufexpl#11.

Requires 'QuitPre' event, only works after Vim version 7.3.544.

Before Vim version 7.3.544 in which 'QuitPre' event was introduced, we
use function 'QuitIfLastOpen' to quit Vim properly. One drawback of this
method is that it does not know the differences between commands ':q'
and ':bd', which means calling ':bd' will also quit the Vim. Since
7.3.544, we use 'QuitPre' event to stop MBE before the Quiting of Vim
actually takes place, and the ':bd' has also been properly handled.