Fixed AlembicSource refresh failure.

Fixes #737.
Fixed errors when AlembicSource filename is "".
Fixed AlembicSource transform test case.
Merge pull request #738 from johnhaddon/alembicSourceFixes

AlembicSource fixes
Added support for multiple types in RSL "coshaderType" annotation.

The coshaderType annotation now accepts a space (or comma) separated list of types the coshader implements.

Fixes #621.
Merge pull request #741 from johnhaddon/multipleCoshaderTypes

Added support for multiple types in RSL "coshaderType" annotation.