Update Stateless.js

Added content-type key to the request post options. Without it Messages.create and DirectMessages.create will not post properly to groupMe.

Added Direct Messaging (index and create) to the list of API calls.
Update Stateless.js

Added a helper function to generate the required source_guid.

Modified the Messages.create and DirectMessages.create to utilize the helper function rather than require the source_guid as a user input.
Merge pull request #3 from AugustBrenner/patch-3

Update Stateless.js