Refactors setting of matrix objects, removes unnecessary variables

* extends Program class by adding methods to get all active attributes and uniforms (not used now)
Refactors enabling / setting of vertex buffer input for program
Refactors setting of index buffer

It's not necessary to bind it to a program, but it reads more consistent to do similar to vertex buffer inputs for a program.
Refactor setting of viewport and rendering of triangles
Merge pull request #3 from justahero/refactor-program-and-buffers

Refactors program, use of buffers and rendering