Adds xfce4-terminal as an option

Because I like to use it, and XFCE is included in one of the main Ubuntu distributions, Xubuntu.

I foresee future additions of the following terminals:

- aterm
- guake
- lxterminal
- rxvt
- terminator
- tilda
- urxvt
- yakuake
Merge pull request #180 from benlk/master

Adds xfce4-terminal as an option