Added GafferImage::RemoveChannels.

* Added the GafferImage::RemoveChannels node to allow channels to be removed from the image stream by specifying a selection to keep or remove.
* Added test cases for the above.

Note: This pull request is dependant on the cortex pull request #59.
Added an Enum for the two RemoveChannels modes and updated the test cases for clarity.
Simplified the hashing of the channel names.
Removed unnecessary flags on the ChannelMask knob.
Moved the RemoveChannelsMode enum into the RemoveChannels scope.
Merge pull request #459 from goddardl/RemoveChannels

Added GafferImage::RemoveChannels.
Added a PointConstraint node to GafferScene.

Fixes #482.
Fixed framing error when entering a Box.
Channel viewer now displays channels in grey scale.
Added stuff to make public binaries.

Updated the SConstruct to refer to the latest sensible versions of all the dependencies. Fixed some problem with building those dependencies on linux. Added a little script for making public binaries at IE.
Merge pull request #488 from johnhaddon/publicBuild

Added stuff to make public binaries.
Added Widget.widgetAt() method.

This returns the widget at a specified screen position, optionally traversing up to an ancestor with a specified type.
Fixed TabbedContainer to hide corner widget when tabs are hidden.

The current approach might not interact well with other code which wishes to manage the visibility of the corner widget - for instance it might already be hidden when the tabs are hidden, and then get shown in an unwanted manner when the tabs are shown. The solution would be to stash the visibility when changing it due to tab visibility, but I'd prefer to wait until we have a use case before figuring out the details of that.
Added ability to hide tabs in layouts.

This is achieved via an item on the layout menu, and a Ctrl+T shortcut, which is necessary for when tabs have been hidden and the menu is no longer accessible. This will be of use for building a better standard layout including a Timeline.
Merge pull request #487 from goddardl/ImageViewerRGBASelection

Channel viewer now displays channels in grey scale.
Merge pull request #489 from johnhaddon/tabVisibility

Ability to hide tabs in layouts
Merge pull request #485 from johnhaddon/pointConstraint

Added a PointConstraint node to GafferScene.