Check signatures before applying commit

[joepio]

atomicdata-dev/atomic-data-docs#14

Also relates to #13 and #26

Following implementations required:

• serialize_deterministically to generate a string that can be used to
• validate_signature function on Commit, using serialize_deterministically and a public key from an Atomic Author.

[joepio]

We need some form of cryptographic signatures. I think the ring library is pretty good - many people use it, and the API is well documented.

[joepio]

In order to validate signatures, the Actor must be available at some endpoint. This Actor should contain a public key. The private key needs to be available for the one creating the commit.

[joepio]

Made some real progress on signing. Deterministic serialization is next!