Add BulletShieldEnemy with reflecting shield against Tower's bullets

Add a comment for key code

Add basic How to Play text to html

Add checkmarks and locks to show progress in difficulty levels

Enable difficulty selection menu to have multiple pages

Delete obsolete codes for drawing without EaselJS

Compact large numerical value for readouts on tip

Tweak strength of BeamTower (again)

Missles who lost target simply re-target at nearest enemy

Diverge MissileTower's shooting positions

Wait for caching of images is complete

Embed EaselJS source into source to allow access via https

Add life parameter to Bullet to prevent missiles from living forever

Prevent missile trails from remaining if the missile lives long

Add endurance mode, which never ends until all towers are destroyed