Clarify identifier serialization

I had a tough time interpreting "serialization of the public key", which is hashed to get the extended key identifier. Since the very next section is "Serialization format [for extended keys]" I thought that I was supposed to use the serialization of the /extended/ public key. Then I noticed "ignoring the chain code", so I tried skipping that part of the extended key serialization. Then I realized that what was meant was "the `K` half of `(K, c)`".
Merge pull request #99 from apoelstra/patch-1

Clarify identifier serialization