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