text: Title: [Feature]: Rest methods should take pd.Timestamp types as well for consistency 
  Body: Entities return pd.Timestamp for time related entries, but calls that accept time don't accept that as a valid format.
  
  So doing something like:
  
  ```
  orders = api.list_orders(after='2018-01-01', status='all', direction='asc')
  orders.extend( api.list_orders(after=orders[-1].submitted_at, status='all', direction='asc') )
  ```
  results in a API format error on the after str(pd.Timestamp).
  
  Perhaps this library could check the `isinstance` and use the `.isoformat()` automatically?
  