Added language Isabelle

Isabelle is a generic proof assistant. It is comparables (to some degree) to Coq.

Used in
* diekmann/topoS
* 3of8/sturm
* formare/auctions
* larsrh/hol-falso
* dpthayer/MetaProof

Hello Wolrd example (file must be named HelloWorld.thy):
  theory HelloWorld
  imports Main
  begin
  (*put content here*)
  end