Added missing functionality and documentation.

* The external authorization information and token classes were missing much of their needed accessors.
* The data reading API was missing its start and end date parameters.
* There was some incorrect or missing documentation in all of the classes that has been updated.
Added a validation method for external authorization tokens.

A function that takes into considerations all reasons that an external authorization token may be invalid and returns a simple boolean result has been added.
Fixed a bug with timestamps, and modified how Data objects are built.

Timestamps were not being properly read or serialized. This has been fixed to ensure that they always appear as ISO8601-W3C strings.

The Data class required the corresponding schema, when, in reality, all it needed was the ID and version of the schema.
Add the fitbit api jar.
Add the beginnings of a Fitbit shim.
Hardcode some data to be returned from the shim.
Add in the Fitbit API and enough shim code to fetch a single steps
datapoint.
Add in a missing dep for fitbit4j.
Fix up the access token secret stuff.
Clean up the steps fetching code and get ready for adding more data
types.
Expose some more data fields for activity.
Add some sleep data.
Fix up everything for the latest upstream changes.
Fix the returned schema IDs to have the prefix attached.