Sharing datasource/handler

[McCrea]

• Provides a JSON datasource for getting a list of users and the permissions they hold for a given file
• Provides a handler which takes a JSON action list representing permissions editing and uses it to perform tasks requested by the client-side. Actions currently supported are:
  • Add user to file
  • Drop user from file
  • Edit user's permissions for file

[hazeld]

This looks good.