Add chain object

[iamdefinitelyahuman]

What I did

• Add the chain object
• Move time travel / snapshot / mining methods from rpc to chain, mark rpc methods as deprecated
• Updates to documentation

Closes #611

How I did it

• Chain is a singleton that emulates list syntax to provide access to web3.eth.blockNumber and web3.eth.getBlock
• the revert registry has been moved from rpc to state
• Chain.get_transaction provides a public API for generating TransactionReceipt objects when the tx didn't originate in Brownie - imo this is much nicer than having to manually import TransactionReceipt

How to verify it

Run tests. I added some new cases and converted over quite a few existing ones.