Introducing sub-classable Director singleton.

Two changes made to allow the override of
CCDirector::purgeDirector()

* Introduce the constructDisplayLinkDirector() which can be implemented
in a client application to construct a child of the display link director.

* The protected method purgeDirector is made virtual.
Merge pull request #6 from lumoslabs/palange/subclass_director

Introducing sub-classable Director singleton.