Crowdsale is a base contract for managing a token crowdsale, allowing investors to purchase tokens with ether. This contract implements such functionality in its most fundamental form and can be extended to provide additional functionality and/or custom behavior. The external interface represents the basic interface for purchasing tokens, and conform the base architecture for crowdsales. They are not intended to be modified / overriden. The internal interface conforms the extensible and modifiable surface of crowdsales. Override the methods to add functionality. Consider using 'super' where appropiate to concatenate behavior.