Duplicate verification code of Pointcheval Sanders signatures

[indomitableSwan]

Right now we have duplicate code in verification of Pointcheval Sanders signatures and blinded signatures. Verification of blinded signatures is required as part of the proof of knowledge of a signature.

We can eliminate duplicate code if we view the verification of a blinded signature for a given public key as a regular signature verification using a related, extended public key that appends the generator g2 to the y2s.

This issue potentially relates to #149. Since a public key only needs elements from G1 to enable blind signing, we do not need G1 elements in the extended public key in order to verify a blinded signature.