Added the //addleaves command.

This adds leaves to logs to form somewhat natural-looking trees. Turns this:
![addleaves_1](https://f.cloud.github.com/assets/4388386/2415894/4efd51e0-aafb-11e3-8dc1-1d53aa468cd5.jpg)
into this:
![addleaves_2](https://f.cloud.github.com/assets/4388386/2415895/55b8e012-aafb-11e3-9763-8d16a9f4445b.jpg)

And turns this:
![addleaves_3](https://f.cloud.github.com/assets/4388386/2415898/5d89c45a-aafb-11e3-94a7-20830fe1c74d.jpg)
into this:
![addleaves_4](https://f.cloud.github.com/assets/4388386/2415901/62de3c7e-aafb-11e3-9545-516337e69adc.jpg)