Import Export User Interface

Implement the Import Export user interface using the /export/ and /import/ action.

The /export/ action should ask a few questions:

x List spaces to export (default to all)
x Include Versions (default to yes)

The export should call the package plugin in "backup" mode and export as a zip.
You can see some example code in velocity calling the package plugin here
http://www.xwiki.org/xwiki/bin/view/Main/ImportExport?xpage=code