Had targets include the src directory rather than other subdirectories.

This is to avoid naming collisions between the subdirectories. Also keeps
things explicit... For instance, server/Paddle.h now includes "common/vector.h"
rather than "vector.h". CMakeLists.txt files have been updated to reflect this.
The Paddle now has a default "null" id. + Documentation.

Paddle::id now has a default of 0.
The PaddleID type alias has been documented.
Changed the Server member function name from makeClient to makePaddle.

This is because it no longer returns some key. It returns a paddle ID. This
makes more sense, to me.
Added a width and height property to the Paddle struct.
Added a Really Simple Server implementation.

Basically: Paddles move!
Since server/ConcreteServer.cpp is the first cpp file in the server folder. I
also added a new target which links to ppmcommon.