Merge pull request #856 from esparta/master

[en] Cleaning up Parallel Programming section
21'000 !
Isabelle/HOL

Link to current version as described in http://www21.in.tum.de/~nipkow/LNCS2283/
Merge pull request #857 from diekmann/patch-1

Isabelle/HOL
Concrete Semantics - A Proof Assistant Approach
Merge pull request #858 from diekmann/patch-2

Concrete Semantics - A Proof Assistant Approach
[en] Adding book on Open Source section