Major release with new provider-format

Breaking changes in 2.0.0 from 1.5.0:

• New provider format which is just much easier to work with and to
  debug if something breaks. It's just a regular alist now without
  macros and not any magic functions running when declaring the list.
• Rename webpaste-paste-text and webpaste-paste-text-to-provider to
  be in "private scope" since they aren't meant to be used by the end
  user. So if you use them, just add a second dash after webpaste.

Provider news since 1.5.0:

• The integration tests still runs once a week, but now we also run
  them for pull requests and all commits in general. One change is that
  we don't run them for all versions of emacs but one (25.3). Github
  gists is also excluded since they only return 403 Forbidden when you
  paste from travis anyways.

New options since 1.5.0:

• You can now add hooks to the return url function to have custom
  handlers for returning the URL to you. Simply add-hook to
  'webpaste-return-url-hook.

Project news since 1.5.0:

• 15 commits by one contributor and documentation updates.
• Lot's of restructuring and changes overall.
• Better provider format
• Repairs of the build system