Removed deprecated IECoreGL::Selector::loadIDShader() method.

The pushIDShader() method should be used instead.
Added IECoreGL::Selector::postProjectionMatrix() method.

This is useful if the OpenGL projection must be changed after the construction of the Selector (e.g. to use an orthographic projection for overlays).
Merge pull request #316 from johnhaddon/glSelector

Selector