Add Timestamp support for use as TTLs

Merge pull request #8 from pquerna/pq/time_expose

Add support for exposing timestamps as unix seconds, ms, or ns.

v0.1.2: Merge pull request #7 from morgabra/morgabra/DynamoVersion

Generate an optional 'Version() (int64, error)' func

v0.1.1: Merge pull request #6 from morgabra/morgabra/fix_reflection_import

Regenerate dynamo.proto at 'dynamo/dynamo.proto'

v0.1.0: Merge pull request #5 from pquerna/pq/moar_key_iteration

Add const keys, and also write key funcs for compound fields