Support to other Databases

The project has PostgreSQL as a requirement because JSON columns are used, both on `Apicasso::Key` and `Apicasso::Request`. When using other DBs this is achievable through AR serialization and a common text field.

Inspect Apicasso::Key and Apicasso::Request to find how PostgreSQL JSON columns are currently used. Read the relevant Active Record serialization behavior and test database compatibility; done means the engine no longer requires PostgreSQL for these fields and existing behavior remains covered.

索引モデルが issue の本文から書いたものです。