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
Isabelle language - fixed lexer and added sample

Also, Isabelle is very polular in academia.
See for example http://scholar.google.de/scholar?q=isabelle%2FHOL

In around 40 days, the seL4 microkernel [1] with its
Isabelle proofs is (probably) released on github [2].
[1] http://sel4.systems/
[2] https://lists.cam.ac.uk/mailman/htdig/cl-isabelle-users/2014-June/msg00011.html
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Merge pull request #1280 from diekmann/patch-1

Added language Isabelle