Replace last of Content's ECDSA.verify / sha256 with AsyncCrypto (#19)
Replace last of Content's ECDSA.verify / sha256 with AsyncCrypto (#19)

- Use new AsyncCrypto.verifySignature (#24 added)
- Removes last expensive main-thread crypto from Content and closes #19

dattnetwork/datt,ryanxcharles/datt,yoursnetwork/yours-core,yoursnetwork/yours-core,ryanxcharles/datt,dattnetwork/datt