Added ImageReader::supportedExtensions() method.
Delayed module imports in test app.

When running "gaffer test ModuleName", the other modules are no longer loaded. This can be useful when running a subset of the tests in an environment where GafferUI won't import due to the lack of an X connection (over ssh for instance).
Added Viewer.view() method.

This returns the current view, which allows its settings to be modified - they are exposed simply as plugs on the View class itself.
Added SceneReader::supportedExtensions() method.
Added workaround for Qt OpenGL problem on OS X.

This would manifest itself as "invalid framebuffer" errors leaking into the IECoreGL and PyOpenGL code and being reported there, causing various forms of havoc.

Fixes #404.
Fixed "invalid drawable" error messages on OS X.

I had hoped that this fix would make the previous commit unnecessary, but testing shows that they address different problems and both are needed.

Fixes #396.