CONTENTBOX-399: Export/Import ContentBox Site as .cbox package

This contains code to export/import entire ContentBox site (or pick-n-pack) as a .cbox package.

Luis, I played around with the interface for the import/export screens. Let me know if you like it--otherwise, if you prefer the more tabbed look, it's easy enough to revert.

![exportview](https://f.cloud.github.com/assets/691607/1648186/c7737728-5978-11e3-9bb6-e9cf74a31a33.png)
![importview](https://f.cloud.github.com/assets/691607/1648187/c775276c-5978-11e3-85fa-4cdbea57c70e.png)