Custom Pilot/Mech Images

Hello again,

Now that (it seems like) the really critical bugs have been dealt with, I can continue on the path to a 1.0 release. Today's update:

• Fixes an issue where negative mech stats could be rendered like +-X instead of just -X
• Removes the default pilot portraits and avatars
  • These (or a form of them) will come back as an additional downloadable package
• NEW FEATURE: Custom Pilot Images
  • Save and use your own pilot portraits and avatars
  • Any web-supported format is also supported by COMP/CON
• NEW FEATURE: Custom Frame Images
  • Save and use your own mech images
  • Frames default to official art (or an empty image, if there isn't any yet). You always have the option of switching back to the default.