Find bug in MnemonicCode.toSeed()

[stevenroose]

The toSeed() method of MnemonicCode seems to be incorrect.

This can either be our fault, or due to an incorrect PBKDF2-SHA512 implementation of cipher.

[stevenroose]

I found one bug in the toSeed code but it still doesn't make it work as it should.

e85d12b

[stevenroose]

This can possibly be related to PointyCastle/pointycastle#92

[stevenroose]

Needs testing now that PointyCastle/pointycastle#92 is fixed