v0.19.0

• Remove testify dependency from our tests
• A new extension x/cron is added. It allows to configure weave application
  to be able to schedule messages for future execution.
• Proposals created with x/gov are having their tally executed automatically
  after the voting time is over. This is possible thanks to x/cron extension.
• Add owner index to bnsd x/username to be able to query tokens by owner.
• Allow empty targets in bnsd x/username to enable name reservation.
• Allow to update election quorum.
• Add self referencing address attribute to entities aswap.Swap,
  escrow.Contract, distribution.Revenue, multisig.Contract gov.ElectionRule and
  paychan.PaymentChannel.

Breaking changes

• weave.Ticker interface was updated.
• Add owner index to bnsd x/username to be able to query tokens by owner.
• Allow empty targets in bnsd x/username to enable name reservation.
• Username address type in x/username extension was changed from []byte to
  string. Instead of base64 encoded value, a valid string is stored as the
  address.
• Some of the query paths in the x/gov package were updated to follow the
  naming convention.
• gov.TallyMsg is no longer available. Tally is created automatically when
  the voting time is over.