Major overhaul for GLMeshItem, MeshData classes

   [ Note: These APIs have changed significantly. ]
  - MeshData and GLMeshItem now operate on numpy arrays instead of lists.
  - MeshData can handle per-vertex and per-triangle color information
Added GLSurfacePlotItem class based on new GLMeshItem
GLGraphicsItem now has per-item support for customizing GL state (setGLOptions method)
Added several new shader programs
Added new examples:
   GLIsosurface
   GLSurfacePlot
   GLshaders