Fixed two homing bullet issues

Homing is now reflected network-wide and the player can now
successfully disable the lock-on effect by middle-clicking. This fixes
issue #11. https://github.com/doctorspoof/SpaceShooter/issues/11
Fixed explosive force on asteroids not syncing

The explosive force is now applied then the game will wait for one
FixedUpdate() to synchronise the asteroid over the network to ensure
the force is applied.
Fixes asteroid velocity issues over the network

Now when asteroids spawn they wait for a FixedUpdate() before
synchronising the velocity to others. This makes asteroid positioning
over the network much more accurate too.
Merge pull request #18 from doctorspoof/peter-branch

Merged asteroid and explosive force fixes