Clear message on setup() if the partitioning schema is not found
Implement monthly partition schema on timestamp[tz]
Dropped type field from partitioning schema

The field is now only on the vtable, so that there is a single
description of each partitioning schema.
More strongly typed schema implementation methods

Avoid casting the key to text and then parsing it back to what it was.
Added range partitioning by day
Added a few TODO points

- setup() should take a string containing param=value pairs.
- one parameter shoud be used for timezone in partitions
- one parameter should be partition search (asc, desc, bisect)