Stage.CreateNew - add support for LoadPolicy

[murphyeoin]

Description of Issue

LoadPolicy had no persistent effect on the stage, which is why it was only a parameter to Open(). But now that it has (late last year) persistent effect on added arcs, we probably should add it as a parameter to CreateNew() to make it consistent with other Stage methods
see http://graphics.pixar.com/usd/docs/api/class_usd_stage.html#a7a661b8f729152c673078fbbafe29a46 for the enum in question
For completion, should probably add to all CreateNew() and CreateInMemory() methods

Steps to Reproduce

want to be able to do e.g

Stage.CreateNew(const std::string& identifier, InitialLoadSet load=LoadAll))	

System Information (OS, Hardware)

Package Versions

Build Flags

[jtran56]

Filed as internal issue #149846.