Environment: Add documentation
Remove Bullet from the Bakge library
The reasons for this are pretty straightforward. Bullet is a large and
diverse piece of software that would be foolish to abstract over. Users
should be able to set up their physics worlds how they want it. It will be
very straightforward to extract mesh data from bakge::Mesh objects, which
can then be used to create colliders. And all Shapes classes will provide
the necessary API to extract dimension info used to create primitive
shapes in Bullet or any other physics library.

This has taken some thought but I feel it is best to leave the choice of
physics engine up to the end-user. The GDK engine will still use Bullet
for its physics, however.
mainpage.dox: Update main page docs
Removing Renderer interface and classes
Removing useless README
Removing unused shape classes and fix examples
These will be re-added for a later version
Fixing audio class file descriptions
Crowd: RotateMember accepts constref Quaternion
Remove network classes and code
Build: Fix OpenAL messing with CMAKE_BUILD_TYPE
internal/Debug.h: Fix include guard comment
internal/Debug.h: Add error namespace
internal/Debug: Add GLFW error handler
Texture: Bind error checking for debug builds
Texture: Add SetLocation method
Also moved debugging for texture units into this new method; that way it
doesn't have to happen every time Bind is called
Texture: Fix unsigned mismatch in SetLocation
Texture: Add link to glActiveTexture reference
Texture: Add GL error checking to Create
Utility.h: Log no longer inline
Texture: Update class description