Updates to persistence

[iamdefinitelyahuman]

What I did

• Deprecate existing Contract.__init__ functionality in favor of class methods:
  • from_abi: create manually from an ABI
  • from_ethpm: create from an ethpm manifest
  • from_explorer: create by querying etherscan (!)
• Store persistent data for live networks in an sqlite database at ~/.brownie/deployments.db
• Allow aliasing and restoring persistent data between sessions via Contract.__init__