Other databases

Moe uses sqlite by default, but the underlying database code uses sqlachemy, a database abstraction layer. Therefore any database backend is, in theory, able to be used. However, there are several (known) potential limitations/conflicts with using another database backend:

1. Track and album fields aren't defined with character limits.
2. Support for the `regexp` operator used for regex queries.
3. Parsing sqlalchemy `IntegrityError` exceptions.
    * sqlalchemy throws an `IntegrityError` for multiple database errors such as failed unique constraints or foreign key constraints. Moe currently parses this error to throw a more specific exception, however, the `IntegrityError` error message is somewhat database-specific in format.