test on 3.7
fix collections deprecation warning

Clarify Group invocation in quickstart

For new users and, as noted in #663 , especially those whose
introduction to click may coincide with their introduction to python, it
may not be clear that a Group is a callable. More specifically: the
"root" of your command tree is the callable you actually want to invoke
to run your CLI.

closes #663

Add a docstring which defines the behaviors of the class, including
default formats supported, usage of ``datetime.strptime`` and "first
successful parse wins" behavior.

Also add '%Y-%m-%d %H:%M:%S' to the default formats, as it is at least
as commonly seen as '%Y-%m-%dT%H:%M:%S'.
Minor test fix to handle the new format.

Add ``click.DateTime`` to changelog.