Start DynamoDb connection class

[rwblickhan]

Create a class to encapsulate a connection to DynamoDb, using the boto3 library. This will eventually be called from the database facade created in #30. For now it's probably enough to stub out some of the major methods, and possibly start methods to convert instances of our data model into DynamoDb equivalents.

[rwblickhan]

Actually creating the connection to DynamoDb may have to wait for #24, since we don't currently have a DynamoDb table set up.

[chuck-sys]

• document user table
• write tests that require local session of dynamodb (temporary)
• make the tests run automatically (fun fun times)
• delete_user function that deletes a user given a slack_id