Define standard and implement prototype for async calls

[spalladino]

Async calls are calls executed by a contract that have their execution continued in a subsequent transaction. A good example use case is contacting an oracle: a contract requires a piece of information from an oracle, so it files a request, to be fulfilled later by the oracle once the data is injected from a trusted party off-chain.

Evaluate other use cases, and help define a standard for such calls.

[spalladino]

This idea came up during EthDenver, from a team building a bridge from ethereum to wolfram alpha