Significant changes:

* Refactoring all around; moved most initialization into a function so that it can be reused
* Add a setOptions function that takes a new options object, replacing the current options
* Add a setStartDate and setEndDate function to change the date range after initialization
* Add a remove function that removes the picker from the DOM and unbinds its events and data
* Trigger events when the picker is shown or hidden, and when the apply or cancel buttons are clicked
* Add new singleDatePicker option to show a single calendar for picking one date
* Update the examples file to include examples of all the new features

I think that should cover all the most popular issues. Hopefully I haven't introduced too many bugs by updating many things at once.