Sequence dispatcher

[radix]

Add a SequenceDispatcher dispatcher, which is similar to EQFDispatcher, except that it requires the inputs to be in the order that the intents are actually performed.

This allows meaningful testing of code that performs equal intents multiple times, when the order and sequencing of intents matters.

[cyli]

Nitpicks:

1. It might be useful to add another test case for if effects are out of order, as opposed to not in the sequence list at all - I could imagine an implementation where these would pass if SequenceDispatcher behaves just like EqDispatcher.

EDIT: this can probably just adding "hello" as a second item to the sequence passed to the dispatcher in test_mismatch, since in that case "hello" would be in the dispatcher, just not at the top of the list.
2. Would it be useful to document a return value of None if there is no matching sequence? Maybe this is just assumed to be the default behavior if there is no dispatcher for an intent...

[cyli]

LGTM if travis +1's :)

[radix]

Thanks very much @cyli!