Revamped automation.

- Hard dependency on Python 3.6.
- Migrated from tox to pipenv.
- Minimized diff between pelicanconf.py and publishconf.py (dev and
prod).
- Updated .gitignore.
- Simplified some Fabric tasks, removed unused tasks (requires Python
3.6).
- Added local dev server with auto-regenerated files Fabric task.
- Removed develop_server.sh and Makefile, Fabric does everything.
- Changed README from reStructuredText to Markdown, updated content.
- Added .envrc for easy pipenv support for development.
- Reran pelican-quickstart from the latest release, applied changes.
- Corrections for working links in local dev, no longer needs
pelican.serve, Python http.server works just as well.