Extended the recognition of default types to date and datetime in ISO
format. Fixed a bug when running plac scripts from Jupyter notebooks,
signaled by https://github.com/ursachi and https://github.com/rkpatel33.
Moreover, at user request, removed a Python 3.7 deprecation warning,
added a LICENSE.txt file and a Quickstart section to the README. plac
is tested on Travis for Python 2.7 and 3.4+ but it should work also
for all the other 3.X releases.