[neo-project/neo] Store contract ABI on the blockchain (#343)

I thought about this Erik… but the problem is that you spend gas on a regular invocation, and this is the kind of thing that should be provided publicly (in fact, using the ABI you will be able to know which methods to invoke…. including this «abi» method). Do you think it’s possible to add an optional argument in Contract.Create, verifying if stack is empty?