Default attack now handles power/skill choice with fire

Fixes #1334.

I've decided to deal with this issue by expanding the capability of the default attack in the following case:
- attack is one-on-one
- fire assistance available
- the only possible attack types are Power and Skill

In this case, the default attack will always choose Power, since this allows both exact firing (like in the Skill case) and over-firing. Thus, there is no more need for a failure message in this case.

I believe that the failure message is now valid for all remaining cases, but if I'm wrong, let me know.

http://jenkins.buttonweavers.com:8080/job/buttonmen-blackshadowshade/455/
I don't like this --- what if later code is added which looks at the values in $validAttackTypeArray after this function is called?  I guess i have a couple of thoughts, namely:
* If i understand the code as it stands right now, i think `BMAttack::possible_attack_types()` will always return Power before Skill anyway.  So you could just skip modifying the attack types and return true, and the right thing would happen.  You might not want to count on that, but i'd do that by putting in an assert() that `$validAttackTypes[0]` is the one that is Power, since our expectation is that that will already be true.
* If you really feel the need to modify the array, set it to `array('Power', 'Skill')`.  But modifying the array seems unnecessary to me.