New Xfig formula for homebrew

Xfig is a vector drawing programm for linux. 
Xfig is quite usefull as gnuplot can export in the xfig format, and xfig can produce high quality figures for publishing by exporting to the .eps format. It can export even to many other common formats.
Xfig needs transfig, for which also a formula is included in this pull request.
In the closed Issue #4108, it was asked to include xfig into homebrew.

Please let me know if this brew is usefull for the comunity, or if some stuff needs still to be checked.
On my system (OSX 10.6.8, Xquarz 2.7.4) it runs so far without problems.