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/