Move scraper base classes to SayIt.

This module used to live in sayit.mysociety.org, but as it is of
more general use we're shifting it to sayit. It shouldn't be considered
to be part of a stable API at sayit though, so I've added a note to
that effect in the docstring of the module.

These scraper helper classes need a couple of extra python packages.
As these aren't necessary for every user of SayIt, we've put them
in a separate requirements file

requirements-scraping.txt.
Migrate from popit-django to django-popolo.

This involves six migrations:
* Add new PopoloSpeaker table
* Add speaker_new columns to Speech and RecordingTimestamp
* Migrate all speaker data from Speaker/Person tables to PopoloSpeaker
* Remove the old Speaker table and speaker columns
* Rename the speaker_new columns to speaker
* Rename the PopoloSpeaker table to Speaker
Merge remote-tracking branch 'origin/move-scraper-base-classes-to-sayit'