Adding NIST P curves and testvectors included.

[kristoffer-paulsson]

I made an implementation with P-192, 224, 256, 384 and 521 based on your code, I also sorted the testvectors from NIST and wrote initial UnitTests but not complete. I think that if the NIST Prime Field curves can be certified in your code, then you should know wether you can rely on the Bitcoin curve already present.