Make mock mssql adapter handle server_version correctly
Make Dataset#schema_and_table handle literal strings correctly

Previously, literal strings could be turned into non-literal
strings and thus quoted literal by the identifier quoting code.
Add Sequel::ConstraintViolation exception class and subclasses for easier exception handling

Previously, Sequel raised all database errors as
Sequel::DatabaseErrors, except disconnect errors.  This adds support
for adapters to use regexps to scan exception messages and raise them
as more specific types.  5 new exception classes have been added:

* Sequel::ConstraintViolation (generic superclass)
* Sequel::UniqueConstraintViolation
* Sequel::ForeignKeyConstraintViolation
* Sequel::CheckConstraintViolation
* Sequel::NotNullConstraintViolation

The framework adding support for this is generic enough to handle
other exception types, including adapter/database specific types.

Update RDoc for some exception classes and minor refactoring to
database_spec.rb while here.
Add Sequel::Postgres::ExclusionConstraintViolation

PostgreSQL supports exclusion constraints, which are similar to
unique constraints but instead of disallowing equal entries, they
disallow overlapping entries.  This gives a unique exception
class for such errors.
List CUBRID on website index page