Add setup tag

This is needed for Debian and Ubuntu.

"ibus-sunpinyin" Debian package installs "ibus-setup-sunpinyin" to /usr/lib/ibus-sunpinyin/ ; "ibus-setup" cannot find the setup program automatically and thus render "Preferences" button gray.
http://packages.ubuntu.com/raring/amd64/ibus-sunpinyin/filelist

Add explicit <setup> can solve this problem.
Merge pull request #37 from maxiaojun/master

Add <setup> tag to IBus engine meta-data