BIP 32 proposed changes

Hello all,

based on some feedback, I've created a pull request with a rewritten version of BIP 32, hopefully making it more readable: * Don't reuse the terminology 'public' vs 'private' for the alternate derivation scheme which doesn't allow computing child public keys from parent public keys, but call them "hardened". * Add explicit type conversion functions. * Don't use the ' suffix to mean "alternate" in two very different meanings (use the 'h' suffix to mean hardened, and use CKDpriv and CKDpub for the derivation functions that operate on private and public keys). * Several smaller changes.

These are just documentation changes, the semantics are unchanged.

Comments are welcome, see https://github.com/bitcoin/bips/pull/12.