added subtraction logic to enough()

+ added subornation logic to enough(). We do not want to mix the public
subtract() call with this method. As doing so would mean that either a)
subtraction can go below 0 (this should not happen) or b) we break this
function with exceptions. This is a work around for these scenarios.

Mage.Tests/src/test/java/org/mage/test/mana/ManaTest.java

@@ -1,6 +1,5 @@
 package org.mage.test.mana;
 
-import junit.framework.Assert;
 import mage.Mana;
 import mage.constants.ColoredManaSymbol;
 import mage.constants.ManaType;