Style Correction

Fixed a few instances where variables were named as variable_name
instead of variableName. Renamed the Tower_Defense.java as well.
Refactor attributes from Turrets, change total_cost to totalCost

Removed range and color from turrets as it’s already defined by the
base Turret.

Changed total_cost to totalCost and removed the buy method as it’s
already being used in the base turret.
Merge pull request #2 from kevinlondon/master

Refactor Turrets and base TowerDefense class