text: Title: dfs `cutoff_time` should take a datetime string
  Body: - As a user, I wish featuretools `dfs` would take a string as cutoff_time aswell as a datetime object
  
  #### Code Example
  
  ```python
  fm, features = ft.dfs(entityset=es,
                        target_dataframe_name='customers',
                        cutoff_time="2014-1-1 05:00",
                        instance_ids=[1],
                        cutoff_time_in_index=True)
  
  ```
  
  as well as 
  
  ```python
  fm, features = ft.dfs(entityset=es,
                        target_dataframe_name='customers',
                        cutoff_time=dateutil.parser.parse("2014-1-1 05:00"),
                        instance_ids=[1],
                        cutoff_time_in_index=True)
  ```
  