Fix some rdoc text in the pg_inet extension
Add link to modification_detection plugin to website
Add pg_enum extension, for dealing with PostgreSQL enums

This is simpler than most of the other pg_* extensions, since
PostgreSQL returns enums as string values by default.  The
most useful part of this is returning the possible enum values
in the schema hash, since otherwise you have to query the
system catalogs to get the information.
Fix regression in timestamps and table inheritance plugin where column values would not be saved if validation is skipped (Fixes #839)

This adds a _before_validation private hook that is called both
before valid? and in save before validation is done, even if
validation is skipped.  This ensures the hook is always run
during save, and always run before validation.  It is useful
whenever plugin code needs to set column values on the object
that it might be validating, but must be set before save even
if not validating.

This regression occurred in 4.11.0, and since that isn't the
latest release, this will not be backported and a new gem
will not be released, per Sequel's usual policy.
Make composition and serialization plugins support validations on underlying columns

This changes the composition and serialization plugins to use the
recently added _before_validation hook, so that validations on
the underlying columns are allowed.  This should work better
when the auto_validations plugin is used and there are NOT NULL
constraints on the underlying columns.
Update documentation for pg_* extensions (Fixes #838)