Make heavier enemies generate larger explosions on death
Add BeamTower, a counterpart of BeamEnemy for the player
Use gradation to improve graphics of beams
Remove redundant and error prone serialize methods for Towers
Make BeamTower sticky; i.e. less easily switches target

The beam reaches instantly and switching target to nearer one when
the tower already has one has little merit.  BeamTower is best focusing
on the same target.  On the other hand, towers with projectiles are
better switch target to nearer ones.
Make HealTower increasingly efficient as level goes up

Also prevented it from over-healing, i.e. making healed tower's health
higher than its max health.