v0.7

v0.7

Some enhancements implemented:

#53 exploding creeps
#63 multiple creep waves
#80 Laser shooting sound
#81 creep death sound
#84 Tutorial level(s)
#87 different enemies should cause different amounts of damage on the base
#100 Mission briefing screen

Some of the bugs fixed

#122 a tower placed on the right edge doesn't seem to shoot
#135 Removing rocks should not decrease the number of towers