ECP group cofactor

[davxy]

Add cofactor (h) to curve group structure.

Mostly used as the generator order.

For example ecdsa keygen is currently broken for two reasons:

1. we don't produce values within the scalar field r = n/h
2. we don't exclude trivial scalar values (i.e. 1 and r-1)

Curves included within the library have all cofactor h=1, but we should be more generic to allow the user to plug-in other curve params (e.g. secp112r2 has h=4)