API changes.

- Standard curves definitions moved to separate file
- jk.pub() function added to instanciate Pubkey
  from hex representation on a named curve
- jk.std_curve() function added to instnciate named curve
  curve objects are cached
- Curve.comp_modulus() doesn't change internal state of object
  any more
- Standard curve 257 renamed from DSTU_B_257 to DSTU_PB_257