RabinBenOr doesn't need BerlekampWelch Decoder

as the hashes make sure that only valid shares are returned.
"fix" shortened mac helper

Now this should resemble more closely a 'real' shortened-mac helper.
Still this is only for validation and testing purposes, hopefully noone
will ever use this.
switch from flexiprovider to bouncycastle

which are actually the same, at least we're now one dependency fewer
Merge pull request #18 from andreashappe/master

various cleanups