Virutal DB Specific case insensitive-like helpers

Connection descriptors of specific databases provide details on their
ability
to perform case insensitive like expressions
Build a sql query for specific or generic case insensitive like

If the database provides a specific operator for case insensitive like
(like ILIKE for postgres) use it, otherwise, do a lower() LIKE lower()
to acheive similar results
Add abstract tests for case-insensitive like

All implementing db descriptors run this test in their context
Merge pull request #1 from azavea/topic/caseinsensitive-like

Expand support for case insensitive like