Ability to configure a dialog modal to pop-up when map opens

Hopefully this is helpful for you. We have a use-case where we wanted to add a dialog modal to pop-up when the map first opens. This is configured to use the Access and Usage Constraints field from the webmap properties by default. But this is also customizable in the defaults.js file. 

My first pull request, please let me know if you have any questions or comments.

![2014-11-06 10_03_47-us wildfire activity web map](https://cloud.githubusercontent.com/assets/1638483/4940921/17b88372-65df-11e4-8cf9-eaf7746ba162.png)