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.
Fixed installation to paths starting with "./".
Added skipping of OpenGL tests in travis.

I don't know if it's possible to get OpenGL running in travis yet.
Added travis build status to README
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().
Merge pull request #992 from johnhaddon/travisTesting

Travis testing
Merge pull request #998 from johnhaddon/substitutionEscaping

Substitution escaping
Added 3delight support to travis build.
Fixed RenderManShader compilation in Clang 3.4.
Skipped RenderMan tests requiring a license in Travis builds.
Merge pull request #1000 from johnhaddon/travisDelight

Travis delight
Updated JH personal build config.
Fixed OSLRenderer for compilation for OSL version 1.5.

This version changed the signatures of many of the virtual methods that RendererServices must implement.
Fixed OSLShader for OSL version 1.5.

OSL 1.5 changes many string fields to ustring - we take advantage of the fact that both have a c_str() method so that we can compile for either version with largely the same code.
Merge pull request #1002 from johnhaddon/osl1.5Compatibility

OSL 1.5 compatibility