Added instanced VBO drawing.

Added gl::drawInstanced( const VboMesh &vbo, size_t instanceCount ). You can now optionally draw multiple instances of a VBO with a single draw call.

I have enclosed this new functionality between checks to hopefully deal with compilers or drivers that do not have support for this, but I wasn't able to properly test it. 

https://github.com/paulhoux/Cinder/compare/cinder:dev...vbo_instanced