New Decoders

A *PolySolver* (decoder) can now be passed, similar to a *RandomSource*, to the SecretSharing-algorithms as a constructor parameter. It will then be used for reconstructing the secret.

In addition a completely new *BerlekampWelchDecoder* (with appropriate unit-tests) has been introduced and thereby solving issue #3.

Since this change renders the class *PolyGF256* useless, it has been deleted.