Add a wrapper for use of the batch sendwithus api

These changes make a second class of api (called batchapi) that can delay execution and send a bunch of commands at once to the 'batch' endpoint.

usage is as follows:
```
import sendwithus
batchapi = sendwithus.batchapi(api_key='YOUR-API-KEY')

# below commands are queued but not executed
batchapi.create_customer(email='test+batch+1@sendwithus.com')
batchapi.create_customer(email='test+batch+2@sendwithus.com')
batchapi.create_customer(email='test+batch+3@sendwithus.com')

# below command executes the 3 queued commands.
batchapi.execute()
```