Header fillename convention.

[Adrian Ratnapala <s369625@student.uq.edu.au>]

Our coding style guide leaves it open as to whether to put the
subproject prefix onto the names of headers.  We shoul make a
decision one way or another.

PRO prefix:  There is still a possibility of namespace clashes without 
             them.

CON:         Thanks to the include/PenguinPlay directory the risk of
             such clashes is small, and they are entirely internal
             to PPlay development.