Draft stage.py

@bruth How is this draft of stage.py looking?  Right now the staging databases are named {prefix}{full sha1 digest}, and it might be a minor improvement to name them {prefix}{org_name}{short sha1 digest}, but it may not matter much.  I'm not sure what you had in mind regarding the states, but if there is an error during staging, the data archive is in the Staging state with an error indication.  What condition should usher in the Ready state?  Also, I was thinking I could create OMOP constraint drop and creation scripts, which we could use to test the foreign keys, etc.

Signed-off-by: Kevin Murphy <murphyke@email.chop.edu>
Error templates can be outside of the class since they are constant.