Token Storage Callback

[robbitt07]

Rational for Changes

In keeping with of sdk approach of an abstracted connector to Schwab API, adding parameters to handle situations where file storage of token is not desirable.

What Has Changed

• Added parameters to Token and Client get_token_callback and set_token_callback to override file storage of token
• Wrapped updated methods for getting and setting token payload
• Updated token input validation to account for updated parameters
• Updated documentation for additional parameters
• Example script for interaction with Redis for token storage