Remove useless default parameter for update and created dates

The default values in:

  class ModelBase(models.Model):
      created = models.DateTimeField( auto_now_add=True, default=datetime.datetime.now(), )
      updated = models.DateTimeField( auto_now=True,     default=datetime.datetime.now(), )

... are evaluated when the class is defined, which means that the
created and updated fields would be always be inaccurate but for
the auto_now=True and auto_now_add=True options.  Those options
override any default that's specified, so the only effect that
the defaults have is to introduce confusing datetimes into the
models field of South migrations.
Propagate introduction of the plural_name field to later migrations

A bad merge meant that the plural_name field was lost in the
models field of migrations 0032 and 0033.  This commit
reintroduces it there so that trying to generate a new
migration doesn't prompt for what to do about plural_name.

This fixes part of #514.
Add a migration that South thinks is necessary

This deals with the issue described in #514 - we don't
really understand why this is required, but our best guess
is that previous migrations were generated with different
versions of South, some of which didn't care about
blank=True.  However, we don't have more time to deal with
this at the moment, so this at least allows me to create
clean new migrations with
./manage.py schemamigration core --auto
Switch to using the master branch of MapIt
Add a command that imports the 2013 election boundaries

This uses the generic mapit_import script via
django.core.management.call_command.
Title-case the names of the 'dis' areas

This is for consistency with the previous generation, which
has all-caps constituency names but title-case county names.
Correct some county names that are wrong in the source data

Fixes #511
Create a ParliamentarySession model and associated migrations

This is intended so that Place and Position objects can
have a foreign key to the particular parliamentary session
that they're valid for.  It also associates that session
with a particular generation in MapIt.  (I'm not sure
about this particular choice, and there's a comment in
the code to that effect.  Currently it's unused.)

This commit also changes the Place model to add a foreign
key to ParliamentarySession and add the migration.
Display the ParliamentarySession for Places in lists of them
Add a method to return ParliamentarySession objects for a PlaceKind

Each Place can have a ParliamentarySession specified for it.
This commit adds methods to PlaceKind that return all
ParliamentarySession objects that are specified for a Place
of that PlaceKind.

There are two versions, one which returns [None] if no
ParliamentarySession objects were found (e.g. as for
places of the "Country" PlaceKind) to make it more easy
to iterate over the sessions in a template.
Add methods dealing with dates to the ParliamentarySession method

This adds:

  - a covers_date method, for testing if a date is during a
    the session

  - a default ordering by the start date of the session

  - add a relative_time method, to return "Past", "Current"
    or "Future" for the session
Change the place_kind view to deal with parliamentary sessions

Now, instead of having multiple PlaceKinds to represent
constituencies at different points in time, we just have
one PlaceKind for 'Constituency', and Places of that
PlaceKind have a particular ParliamentarySession.

This commit changes the place_kind view to iterate over
sessions for a particular PlaceKind rather than just
each PlaceKind.
Add a command to create parliamentary sessions and migrate places

This command is intended to be called from a datamigration to
make sure that ParliamentarySession objects are created for the
known sessions in Kenya and Nigeria, and make sure that the
PlaceKind and Place objects are updated correctly.

Note this uses settings.COUNTRY_APP to distinguish which country
this instance of Mzalendo is for, so it also adds a note to that
effect to general.yml-example.
Add a datamigration for Nigeria and Kenya's parliamentary sessions
Merge branch 'boundaries-2013'
Display information about the parliamentary session on Place pages
Add more name corrections to the 2013 boundary import script

Thanks to Jessica Musila for providing the corrections to
county and constituency names.
Add a script to add mapit area IDs to new Place objects for 2013

This is variant of the existing core_match_places_to_mapit_areas
command by Edmund von der Burg, but which is also aware of
ParliamentarySesssions and has some extra debugging output.
Change the import script for 2013 to use ESRI Shapfiles instead of GeoJSON

Annoyingly, the version of GDAL on the production server seems
to be too early to be able to import GeoJSON and it's a pain to
upgrade.  So, instead, I've converted the data to ESRI shapefiles
with ogr2ogr and this commit changes the import script to load
shapefiles instead.
An interim fix for the display of Related Places

The place_detail pages looked confusing since they were
showing both past and future constituencies.  It needs
a bit more thought to decide how to present this right,
but in the interim just indicate "Past Constituency",
"Future Constituency", etc. instead of "Constituency".