Missile micro casting shield on friendly units

[mizzao]

When:

• no enemies in sight, but allies are in sight, or
• enemies are in sight but it's already shielded.

Of course, generally try to self-cast shield first.

[mizzao]

Shield throwing should cast on lowest health unit, including oneself.

[mizzao]

Actually, just prioritize shielding other units because these are usually the tanks who need it. Otherwise, it could land on another missile bot which is no worse off.

[mizzao]

In progress: #54.