Support creating proofs w/o a date

[dlongley]

The LD signature spec will need to be updated to support making created optional, but the API should support it by setting date=false. This is an important use case for blockchain/verifiable ledgers where dates are derived not necessarily by wall clock time but by position in a blockchain. Furthermore, being able to set dates in proofs may be an attack vector for spam in some cases and so it should be possible to prohibit their usage.