BEP Idea: Add support for WebHooks

[ttmc]

This was issue bigchaindb/bigchaindb#1339 but I copied it here and closed it there.

@vrde wrote:

BigchainDB events API (see bigchaindb/bigchaindb#862) should support WebHooks to enable third parties to trigger their own business logic on specific events. This would fit well because many businesses and developerss are using cloud services like AWS Lambda and AWS API Gateway to build their own applications. A WebHook would be able to trigger custom behaviors on their side, in a """""smart contract""""" fashion.