Square refresh token

[allisonking]

Before deploying this, make sure to add an initial credential file in data/wack4square/square_creds.json

Changes:

• Add the token to the db
• Add SquareCredential class to marshal to and from the db
• Refactor the Square class to read its access token from the db instead of the environment variable
• Add oauth token call to Square class
• Implement refresh flow
• Update readme
• tests