Changed include from StaticBuff to ABuff in Traits.php - it does not need to know specifics
Add include of StaticBuff to test/merc.php
Fixed a bug in Merc that was resulting in Buffs being stored in reverse order to the Traits applying them
Merc test script now require's PercentBuff so they may be properly tested
Removed unnecessary require of Merc from ABuff & StaticBuff
Changed the 'robust' example trait to contain 2 buffs - a StaticBuff on strength & a PercentBuff on HP
Adding PercentBuff class (based on StaticBuff) to process buffs that add or remove a percentage to a stat