Fixed Execute->Repeat Previous menu item.

[ci skip] - there's no need to run the travis build for a UI change.
Added scene output to ExecutableRender nodes.

This means they can be seen in the Viewer, SceneHierarchy and SceneInspector.
Added pass-through plugs for ImageWriter and SceneWriter.

This allows them to be viewed in the UI.
Removed unnecessary serialisation of pass-through plugs.
Added pixelAspectRatio option to GafferScene.
Added resolutionMultiplier option to GafferScene.
Added overscan options to GafferScene.

Fixes #979.
Fixed display of '<', '>' and '&' in SceneInspector.

These were causing problems because they have special meaning in HTML.

[ci skip]
Merge pull request #991 from johnhaddon/repeatLastDispatch

Fixed Execute->Repeat Previous menu item.
Merge pull request #994 from johnhaddon/sceneInspectorEscaping

Fixed display of '<', '>' and '&' in SceneInspector.
Merge pull request #993 from johnhaddon/camera

Camera additions
Updated Changes and minor version
Casting the resolution back to ints.

We get warnings without these casts. Shouldn't Travis have caught this problem?
Marked expected failure in GafferImageTest.ImageWriterTest.
Added unit test execution to travis setup.

Not running UI tests at this stage - those can wait until we have the basics working.
Added more test commands to try to understand what is happening.
Removed modules to speed up travis testing.
debugging for install process