Rewrite of Tutorial5 texture loading code

[stuaxo]

Also being new to opengl I had no idea how to fix the error I was getting under tutorial5.

I rewrote the texture loading code based on the original C++ (and some stackoverflow code on using PIL with textures).

The code in this pull request shows the textured cube in linux + mesa.

The error I was getting with the original code is this:

$ python tutorial5.py 
Compiling shader: Shaders/Tutorial5/TransformVertexShader.vertexshader
Compiling shader: Shaders/Tutorial5/TextureFragmentShader.fragmentshader
Linking program
Traceback (most recent call last):
  File "tutorial5.py", line 318, in <module>
    main()
  File "tutorial5.py", line 160, in main
    texture = load_image("Content/uvtemplate.bmp")
  File "tutorial5.py", line 113, in load_image
    bind_texture(texture_id,'MIN_FILTER')
  File "tutorial5.py", line 81, in bind_texture
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP)
  File "/mnt/data/home/stu/.virtualenvs/3d/local/lib/python2.7/site-packages/OpenGL/platform/baseplatform.py", line 402, in __call__
    return self( *args, **named )
  File "/mnt/data/home/stu/.virtualenvs/3d/local/lib/python2.7/site-packages/OpenGL/error.py", line 232, in glCheckError
    baseOperation = baseOperation,
OpenGL.error.GLError: GLError(
    err = 1280,
    description = 'invalid enumerant',
    baseOperation = glTexParameterf,
    cArguments = (
        GL_TEXTURE_2D,
        GL_TEXTURE_WRAP_S,
        GL_CLAMP,
    )
)