Support for columns participating in multiple indices (Fix #258)

Set AttributeUsage.AllowMultiple to true for IndexAttribute and
UniqueAttribute.

Added UniqueConstraintViolationException (SQLite version 3.7.17 and above). 

Added methods to SQLiteConnection that can be used to figure out which
unique constraint(s) would be violated by an insert/update operation. 
This can be used either to perform data validation before saving or to
provide details for error handling.

Added a static class to encapsulate the pragma calls used to get index
metadata. This can be expanded to include other pragmas as needed.