Optional SandGenerators

...ed it back and I created a little "mod" that brings them back. This PR adds a property in bukkit.yml which can enable sand generators. In EntityFallingBlock.java, there is a part in the code which is missing a return; statement. Amaranth added it in and I encased it with an if statement which checks if the allow-sandgenerators property in bukkit.yml is false, if so it runs the return; statement.

Signed-off-by: seifpic <seifpic@hotmail.com>