Rename 'kb_upgrade' command to 'upgrade'.
kegadmin: improve beer type management; add keg management.

Addresses issues #185 & #186.
Closes issue #164.
Properly set uploaded picture extension.
Set keg.finished (data migration).

Default value had been 'True', fixed in 28c1ed9d.
Add Controller and FlowMeter models.

Part of issue #184.
Drop Tap.meter_name and use FlowMeter objects in their place.

Part of issue #184.
Move meter calibration out of KegTap to FlowMeter.

While we're at it, invert the value ratio to ticks_per_ml,
which tends to be a little more human-readable.

Part of issue #184.