Add formal support for Usd <-> Alembic facesets#219

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aferrall wants to merge 2 commits intoPixarAnimationStudios:devfrom aferrall:dev_alembic-facesets

+458-15

Commits on Sep 8, 2017

* Modifications to pxrUsdInShipped to support not only facesets which contain UsdShade-based material assignments,
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Addition of a simple unittest which validates the conversion to Alembic and back
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