playhouse.migrate should use Database.compiler_class when possible
Good catch, thanks I will look into fixing this.
 The thing is that the migrator methods for adding indexes are not aware of the column types, so this doesn't really make sense. Maybe it would be better to subclass `Migrator` and allow the `add_index` operation to accept some additional arguments?

For now I am closing as wontfix.
 You're right, the problem is that migrator has no idea about the column type. As a solution right now, I'm calling the `Model._meta.database.add_index` instead. Tbh, I would just make the migrator method do the same: pass the call to the existent database instance method.

There're some inconsistencies between database compiler and migrator methods.
A good example is the `add_index` method where when using Postgres, array and hstore fields won't get gist index types.

Delegating these methods to the compiler class fixes the issue.
This will also reduce the duplication.
