Normalize rollback argument and doc changes

joplin.core/rollback-db was expecting a string as the number of migrations to rollback.  This was fine when it was being called from the lein plugin, but made calling it directly awkward.  Now the lein plugin converts `n` to an integer first.

Some changes to the readme to make the example :joplin map in project.clj valid and to clarify the difference between :sql and :jdbc migrators.