First pass of updatable result sets

• Broke scrolling functionality out of result-set into scroller classes
• Tests requiring both scrollable & updatable cursors do not pass (but are ignored)
• Cleaned up "query" methods in BasicContext
• Cleaned up "execute" methods in PGConnection
Add SQLXML support to SQLInput/SQLOutput classes
Clean up exceptions in not-implemented methods that are actually not-supported
Initial public API for PGConnection