Crypto-conditions, final revision

I've pushed another commit to the PR that is currently up for review at:
https://github.com/interledger/rfcs/pull/131

This should be the last revision barring in critical changes. Please
provide feedback via the list or on the PR.

Some suggestions such as de-duplication and new signature schemes will be
added in a future draft as we've established these can be added later
without making breaking changes.