Make it easier to test handlers

[adamzaninovich]

The current situation for testing handlers is pretty messy.

Maybe add a Alice.Handlers.Case that provides some nice helpers to make testing handlers easier and cleaner.

[adamzaninovich]

Maybe if Slack was abstracted out into a pluggable adapter, there could be a TestAdapter to help make testing easier. This would also pave the way for a local, repl-type adapter for trying alice without connecting to Slack.