A new explicit FontForge scripting command to cause a crash.

This is going to be useful to verify that a backtrace can be generated
on platforms like windows, osx etc. Having easy routes for users who
see crashes to be able to generate a backtrace should help everybodies
fontforge to become more stable.
Merge pull request #652 from monkeyiq/2013aug/explicitcrashscriptcommand

A new explicit FontForge scripting command to cause a crash.