Rewrote RollCommand.java

Rewrote RollCommand.java to accomplish the following.
1. Fixes errors occurring on rolling multiple dice.
2. Adds error handling code.
3. Adds easily editable maximum values for number of dice/die size.
4. Allows for use of ?d?-? format, instead of having to use ?d?+-?
5. Increases radius of roll messages to 24, up from 16.