Shares

Implemented and documented the new improved *Share*-structure as discussed.
Also added some unit-tests.
(the serialization layout is explained in *serialized.txt*)

I had to modify the *SecretSharing*-algorithms quite a bit to make them work properly with the new shares. The code in this classes has just been modified quick & dirty and is therefore neither more elegant nor better documented.

I will improve the code-quality and the documentation of the SecretSharing-algorithms  in a later changeset though.