Add the feature of synchronization across devices

Summary:
Add the feature of synchronization across devices by using Parse with Facebook
Login. The feature is not production ready, yet, because of the lack of server
code to secure the apps from abuse. However, you can test the feature by using
your own Parse and Facebook developer keys.

The diff also includes a lot of renaming and refactoring to make it closer to
javascript standard, e.g. put every variables and methods into one object
instead of populating the global namespae, use camelCase instead of
underscores, etc.