Remove untested EC curves

This pull request removes the untested curves from `sec.js`, except for `secp256r1`, because unlike the other curves it is tested, and also demonstrates how other curves can be added. While these curves are potentially useful (since our EC/ECDSA code is so good now it might be better than the origin, even for non-Bitcoin uses), these curves are not used by Bitcoin - only the Koblitz curve is used. So we are removing them for now, unless someone wants them in the future.

It is worth mentioning (but was not the reason at all for this pull request), that there is some controversy surrounding many of the EC curves in use, and the possibility that they may have been compromised (by an organization such as the NSA) for purposes of seeding curves which they have determined how to crack. The Koblitz curve was likely deliberately chosen by Satoshi, because she considered this curve to be the least likely to have been compromised in such a way. I'm only adding this information to inform you of the controversy, not to register a personal opinion on it.