Remove the hack to set targeting mode.

KSP 0.24 extended the ITargetable interface with a method to fetch the
mode. Very nice.
Don't let the recycling bins eat asteroids.

Or any ships attached to asteroids.
Make FindResource internal rather than private.

ExLaunchPad wants it too.
Adjust the pad's mass to reflect build progress.

I had completely forgotten that as RocketParts were consumed, the mass of
the vessel would go down, but as the idea is that the consumed parts are
going into the built craft, the overall mass should remain constant. This
"fixes" that. There may be problems with the vessel CoM not updating while
warping, but I don't see a quick solution anywhere, so I'll just wait and
see what happens.

Fixes #43
Put the welded hexcans in the techtree.

Fixes #46
Add a finalize build button.

It turns out that it is better to wait for user action to do the final
build. This allows physics to settle after dewarping, and also prevents a
build happening in a busy scene from causing problems.

Fixes #45