Update to version 02 of the draft specification

draft is at https://tools.ietf.org/html/draft-thomas-crypto-conditions-02

- [x] part of this work should incorporate the test vectors documented at https://github.com/rfcs/crypto-conditions#test-vectors

Some details to double check for:

- [ ] URI normalization: see issues https://github.com/rfcs/crypto-conditions/pull/8 and https://github.com/rfcs/crypto-conditions/issues/12
- [x] add condition validation to tests
- [x] document support for subtypes (which ones) and behaviour of the implementation with respect to unsupported types (types above id 4 are ignored)

#### Things left to be done

- [ ] Test signature operation (see https://github.com/rfcs/crypto-conditions/issues/15)