Add hooks for logging web socket messages

This can be used to capture web socket messages, interspersing them with metadata about the source of such messages. The rippled test suite could be captured (as per http://ndudfield.com/zencoding/rippled-capture) and it could also be useful for logging ripple-client messages for display.

The event names are pretty explicit about the intended use cases.
Merge pull request #21 from sublimator/develop

Add hooks for logging web socket messages