Fixed IECoreGL "primitive:selectable" attribute.

It was only taking effect in GL_SELECT mode and not in the other two selection modes.

Fixes #46.
Fixed GL ID selection in wireframe, outline and points styles.

Leaving bounds mode broken for now because it requires the storage of another shader setup, which would mean breaking binary compatibility.
Merge pull request #211 from johnhaddon/selectionFixes

IECoreGL Selection Fixes