* Moved Asset issue/deissue code from IssueAssetTransaction to Asset business object.
* Added more constructors for Asset using IssueAssetTransactionData or assetId.
* Moved some constants from transaction transfers to business objects (e.g. IssueAssetTransaction)
(They might now make more sense being in Asset)
* Changed some transaction isValid() checks to use transaction's timestamp instead of NTP.getTime()
* New VotingRepository - as yet unimplemented
Really need to rewrite "migrate" and add a ton of unit tests.
