Added a permissions system to the Accounts module.

* Use P.accounts.grant("permission", [callback]); to grant a permission
* Use P.accounts.redact("permission", [callback]); to redact (remove) a permission
* Use P.accounts.hasPermission("permission"); to check if a user has a permission (returns true or false)
* You can set the default permissions of a new account in config/accounts.js (default is one permission: "user")
Added more documentation for Platform's configuration in config/app.js as well as a new "distress" emission system, for high priority messages.