Made SgSchema._valid a private variable.

SgSchema._valid now pushed to private SgSchema.__valid.
SgSchema objects now store a build number hash

Schemas now store a build number hash.  Build hashes are updated anytime the
schema re-builds itself.  This is part of the process for adding live schema
update support to the ShotgunORM.  A schemas build hash can be retrieved with
the SgSchema.buildId() function.