Added support for mixins.

- added mixin module.
- added mixin tests.
added damageable mixin.
typo fixes.
Added phsyical entity

- Converted ShipEntity to inherit from physical.
- Added some comments to BaseEntity.
tiny typo.
fixed duplicate entity creation issue.

Now, we simply return the id of the duplicate entity, and log a warning. If an id is not passed in as part of `entityDef`, then we generate one.
added multiple test accounts to db init script.

Also, no longer require the DB to be erased first.