Document requirement of popolo app.
Update documentation for code changes.

Remove unneeded steps/packages, include Python 3 information.
Add new development page, including packages, CSS, and model changes.
No longer need to install special fork.

Now points directly at fork hosted on PyPI.
Add documentation on easy_thumbnails and local.py.
Document further SOUTH_MIGRATION_MODULES usage.

Both speeches and instances also now have South migrations in the
south_migrations directory, with 1.7 migrations in `migrations`.
Note that some tests fail without FFMPEG.
Note configuration not needed if using South 1.0.
Add Updating Code page.