Use PostgreSQL hstore data type

Using the hstore data type for percentage and group rollouts would significantly improve the speed of and reduce the complexity of Detour, especially in the complex summary views of the admin interface.

Percentage rollouts would be deserialized like:

```ruby
percentage_rollouts: { "users" => "20", "widgets" => "100" }
```

Group rollouts would need to use a key prefix, and would be deserialized like:

```ruby
group_rollouts: { "users|admins" => "1", "users|foo" => "1", "widgets|foo" => "1" }
```

It's worth keeping in mind that this introduces a PostgreSQL dependency for Detour and projects using it.

This might necessitate finding a way to run `CREATE EXTENSION IF NOT EXISTS hstore` outside of a migration in order to prevent forcing project to use a SQL schema instead of the standard Ruby schema.