Add new IP-based time endpoint

[brentru]

This PR modifies the receive_time method to support the new time endpoint on Adafruit IO: /integrations/time/struct.json. This endpoint will return a time based on the IP Address. the time_topics.py example has been renamed time.py to reflect the removal of time-topics in the newest endpoint.

The new receive_time uses the new endpoint and returns the time as a struct_time(https://docs.python.org/3.7/library/time.html#time.struct_time). It also removes the kwarg time from receive_time - previously used to return the requested time type (sec/ms/iso) from Adafruit IO's endpoint.

The previous /time endpoint returned a plaintext response, instead of json. Since the new endpoint returns a JSON response, this PR will remove all plaintext-handling from the library used for the old /time endpoint:

• Remove is_time from compose_url()
• Remove is_time from _get()
• Removal of is_time from compose_url()