Added sandbox.js library

sandbox.js is a port of https://github.com/leighmcculloch/AppSandboxFileAccess to JSTalk
Make Export Selection commands work with the sandbox
Added sandbox environment detection
Use sandbox detection in Export Selection commands
Added init method to AppSandboxFileAccess
Use sandbox class for export functions
Clarify message in sandbox panel
Added note in README about sandbox support. Finally, this closes #9 #22 (hopefully :)