BREAKING CHANGE: require PG 9.3, chronomodel adapter

It's time to remove all the kludge from this library.

Raised the minimum requirement to PostgreSQL 9.3.

Removed the PostgreSQLAdapter hijack, rather the ChronoModel::Adapter
is added as a first-class citizen in ActiveRecord, and this now requires
you to use `adapter: chronomodel` in your database configuration.
Move code around and add a missing spec
Redefine .adapter_name to make it return ChronoModel
BREAKING CHANGE: Use INSTEAD OF triggers instead of Rules
Cosmetic: remove all reference to rules