fix namedtuple in csv reading, and move strip logic to feed.py

The csv output was replaced to namedtuples by @eoghanmurray in
49c20747db88e7e218fb4fc83b61fbc0b37f552d . However load.py was not changed,
and therefore remained incompatible. This is solved here.

Also, the field strip was moved from load.py to feed.py and is
done in an earlier stage.
Allow multiple agencies, in a single gtfs, as well as multiple gtfs files.

Instead of storing agency_id in every table, we store the feed_name. The feed
name is derived from the zip filename or the directory name supplied to `load`.
This allows us to store multiple GTFS files in one database, as long as their
names differ, but it also allows to have more than one agency per gtfs.
Set a default for agency_id in the agency table

sqlalchemy doesn't like a null primary key, even if we have
a composite primary key. So instead of null we put "None".
all columns are unicode