Adds events that can be definied in options.

Previously, there was no way to react to events in stackslider
(ie before flowing, after navigation, etc).  This commit adds
the following events:
- onBeforeNavigate
- onAfterNavigate
- onBeforeFlow
- onAfterFlow

Each event can also be definied in options for greater flexibility.
Merge pull request #3 from ericthelast/feature/add-events

Adds events that can be definied in options.