Added possibility to specify exemption list for entity removal

Addresses issue #240.

Adds parameter to entity removal brush which allows to specify classes which should be ignored when removing entities.

**/b er list-exemptions** or **/b er lex** to list current exemptions
**/b er exemptions +ClassName** to add a class to the exemption list
**/b er exemptions -ClassName** to remove a class from the list
e.g. **/b er exemptions +Pig** will omit all pigs 

The parser can handle multiple +ClassName/-ClassName in a single **/b er exemptions** call.

Class names are automatically prefixed with org.bukkit.entity. If org.bukkit.entity.<ClassName> doesn't resolve into a class it will try to resolve the input without the prefix.