Added mtlib parser for obj files to GLMesh

When load an obj file the mtllib directive is now processed and mtl
files are parsed and store.
Merged in the changes by tanderegg

Had to fix GLMesh.cpp placing the normals check instead of ParseMTL
instead of LoadMesh.
Removed Entity and SceneManager

These classes were obsolete and have been removed.

Entity was in fact just a PropertyMap class named poorly. Thus it was
removed.

SceneManager was added because it seemed like it would be used, but
never was.
Added a flag to CMake for GLEW

Added the GLEW_STATIC preprocessor flag to CMake to remove linker
warnings from GLEW.
Renamed vertex and face

vertex and face have been renamed to Vertex and Face. Also they are now
inside the Sigma namespace.

IGLComponent::NumberElements has been renamed to MeshGroup_ElementCount
and now takes an unsinged int instead of a signed int. Also the
documentation has been better update to reflect what it does now.
Merge branch 'assimp' into develop
Fixed merge
Fixed new naming conventions
Updated documentation for CubeSphere