Fix width of datepicker & prevent it from overflowing window

When the datepicker is aligned near an edge of the screen, showing the calendars may require some adjustment to ensure that everything stays aligned properly.

This is how it looks without this change (ignore the fact that this was v1.0.1 of the plugin; the behavior is the same for v1.1):

![collapsed datepicker](https://f.cloud.github.com/assets/221614/250455/bade4788-8b4a-11e2-9ea1-7be7a40ac9e3.png)

This is how it looks after this change:

![proposed change](https://f.cloud.github.com/assets/221614/250457/c67ea48e-8b4a-11e2-838d-83415a97a296.png)