moved decoder to individual class
implemented BerlekampWelch decoder

also added some unit-tests
Shamir and RabinIDS now using PolySolver

All SecretSharing schemes have been modified to take as an argument in
the constructor a PolySolver, which is then used for reconstruction.
An additional constructor has been introduced to avoid changing the
test-cases.
deleted now unnecessary class PolyGF256
Merge pull request #11 from efrantar-tgm/decoder

New Decoders