Added timed event handling, among many changes.

Code changes:
- Event.java and EventHandler.java have been added for handling
time-based events, see the javadoc for more information.
- A TestEvent class has also been included for reference on how to
extend the Event class if anonymous inner classes are not your
preference.
- Added a class for handling, locking, and displaying mouse input via a
cursor.
- Added a Transform2D helper class, as well as .toArray() functions to
both Transform and Transform2D.
- Props now implement the Cloneable interface and a copy of the Prop can
be obtained via .clone() method.
- New static method in Prop.java - .createCluster(map, props, freq,
center, radius) allows for the creation and dispersion of randomized
clusters across a TileMap, allowing for easier procedural generation of
natural formations such as forests and rocks.
- TileMaps no longer take a height offset in their constructor and now
read the value from the tileset.cfg.
- Reduced the minimum factor for FastNoise.getNoiseImage() to 15f rather
than 30f.

Resources:
- Added a grid overlay tileset.
- Added a few test heightmaps.
- Added a cursor image.
- Added a few tree images as well as the PSDs so you can play with the
Prop code.
- Updated the tileset.cfg files so that height offsets are included to
reflect the code changes.