CrateSpawner changes

Changes:
- Moved all crate spawning logic into CrateSpawner.cs (removed CrateDrop.cs)
- Added support for varying types of crates to be spawned, and specify the chance each crate can appear

I created a test map for CNC which is available [here](http://hamb.in/crate-testing.oramap). The [map](https://f.cloud.github.com/assets/566742/1804293/6af1c424-6c46-11e3-8d95-d2c560c45195.png) contains art for three different crates that were pulled from Sole Survivor.

There might be an unrelated bug - crate tool tips were not working.