Add VertexLayout class, needs comments
Add VboMesh: a drawable collection of geometry using a vertex buffer and (optionally) index buffer
Fix some types in VertexLayout
Fix some types and make functions const where applicable in ShaderProgram
Add example usage of VboMesh to tangram test program
VboMesh now holds a shared_ptr to a VertexLayout object, which will be shared across many meshes
Use reference for iterating over a list
Use a shared_ptr to pass ShaderProgram as a function argument
Clarify the use of 'offset' in VertexLayout
Add destructor for VertexLayout
VboMesh now uses no GL resources if no data is added
Remove some unneeded 'nullptr's
Make argument to 'addIndex' consistent with 'addIndices'
Add example of index usage to test file
Merge branch 'vbo-utils'

Conflicts:
	Makefile
	core/tangram.cpp
	core/tangram.h
Fix merge by adding core/include to android project includes