Build providers infrastructure

[martinfleis]

This PR implements handling of providers JSONs and consists of a few changes:

• include _meta attribute in leaflet-providers-parsed.json storing a description of the file, date of creation and a commit to leaflet-extras/leaflet-providers from which it was created
• leaflet-providers-parsed.json is stored in a pretty format
• add xyzservices-providers.json with templates for additional services not present in leaflet JSON.
• create _compress_providers.py which merges both together and stores them in xyzservices/data/providers.json, which is being shipped with the package and used to create Bunch.
• create Makefile to make compression and updating of leaflet JSON slightly easier.
• move things around a bit again - provider_sources folder with JSONs is now in root

Closes #9