Added pixelAspectRatio option to GafferScene.
Added resolutionMultiplier option to GafferScene.
Added overscan options to GafferScene.

Fixes #979.
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?
Optimised Context::substitute().

Constructing the variable name character by character was unnecessarily slow. We now use the new InternedString range constructor to construct without needing any temporary allocation at all - this gives us a 65% reduction in runtime for a simple test containing two substitutions. Even when the new constructor isn't available, we get a 50% reduction in runtime, presumably because we now have only a single temporary allocation whereas before our use of push_back was causing multiple allocations.
Added '\' as an escape character in Context::substitute().

Fixes #997.
Optimised Context::substitute().

Using a switch rather than a series of if()s gives a further 40% reduction in runtime for testManySubstitutions().