Started work on the very first enemy.

- unmm.core merged into unmm. This is temporary; the plan is to move all the Tmx* classes into a package specifically for level loading, but only the bare-bones functionality for that will be kept in this package. Functionality not required for level loading (ie. boundaries, checkpoints, bullets, etc.) will be moved into one or more classes contained in a "common features" package.
- New class "Enemy" to be used as the base for all enemy types.
- New class "EnemyHardHat", the first concrete enemy type. (Currently just falls to the ground and sits there; no interaction exists with the player or bullets.)