Optimize doubling: secp256k1 has no y=0 point
Explain why no y=0 check is necessary for doubling

Explanation suggested by Greg Maxwell.
Merge pull request #120

e3d692f Explain why no y=0 check is necessary for doubling (Pieter Wuille)
f7dc1c6 Optimize doubling: secp256k1 has no y=0 point (Pieter Wuille)