Implemented multi-tenant unqiue index changes as discussed.  Now supports both computed and duplicated, plus attribute support.
Implemented multi-tenant unqiue index changes as discussed.  Now supports both computed and duplicated, plus attribute support.

# Conflicts:
#	src/Marten.Testing/Acceptance/unique_indexes.cs
#	src/Marten.Testing/Schema/UniqueIndexMultiTenantTests.cs
#	src/Marten/MartenRegistry.cs
#	src/Marten/Schema/ComputedIndex.cs
#	src/Marten/Schema/IndexDefinition.cs

mysticmind/marten,mdissel/Marten,mysticmind/marten,JasperFx/Marten,mysticmind/marten,ericgreenmix/marten,mysticmind/marten,JasperFx/Marten,mdissel/Marten,ericgreenmix/marten,JasperFx/Marten,ericgreenmix/marten,ericgreenmix/marten