Added IDisposable interface to all classes holding native data that must be freed. OpenTK.Toolkit.Init() now returns an IDisposable instance that can be used to cleanup all native data held by OpenTK. This is useful when re-initializing OpenTK (possibly in a new AppDomain), as is the case in the Example browser.
Added GraphicsContext() constructor that takes an existing, valid OpenGL context. This can be used for interop with third-party toolkits which create the OpenGL context themselves.
Removed OpenTK.Graphics.Glu class (reason: deprecated upstream; most functionality provided by OpenTK math; not compatible with OpenGL 3.0+.) OpenTK.Graphics.Glu can be accessed through OpenTK.Compatibility.
Added OpenGL|ES-specific ErrorHelper classes.
Moved OpenTK.Graphics.DisplayDevice and OpenTK.Graphics.DisplayResolution to the root OpenTK namespace (reason: their functionality is not specific and does not depend on OpenTK.Graphics).
Split Graphics*Exception classes into different files.
Made GraphicsErrorException public (reason: necessary for OpenTK.Compatibility).
Improved CreateDummyContext logic to detect and use the context handle of the current thread or a specified handle.
Removed GetCurrentContext() methods from platform-specific context implementations (everything now goes through the relevant IPlatformFactories).
Added OpenGL 3.0 support for GLX contexts.
Added support for creating dummy contexts (useful if you are creating contexts from an external library).
Cleaned up unused and stale code.
