add support for sqlmigrate

[timgraham]

fixes #141

[aclark4life]

Right! So the base wrapper got a get_database method that checks for queries_logged. Then various schema methods e.g. create_model call get_database which either returns a database connection or returns OperationDebugWrapper. In OperationDebugWrapper you determine whether you are logging migrations as they are executed or showing migrations prior to execution, via self.wrapped = self.collection if use_collection else self.db.database. LGTM, thanks!

[timgraham]

The last sentence isn't correct. OperationCollector is for collecting, while OperationDebugWrapper executes.

self.wrapped = self.collection if use_collection else self.db.database determines whether OperationCollector / OperationDebugWrapper is wrapping a database or a Collection.