Loading `db/sqlite3' no longer raises an exception if
the SQLite library isn't found. Instead, `sqlite3-connect'
raises an exception, while `sqlite3-available?' reports
whether it will work.
The dynamic test allows the documentation-help system
to continue to work if SQLite3 is not available. Currently,
though, `raco setup' still insists on using SQLite3 to
build the database of documented tags.
Other major changes:
- pg code now uses only binary format
- pg timestamptz now always UTC (tz = 0), added doc section
- added contracts to most pg "can't-convert" errors
