Update lein-flyway to work with flyway 3.0

Rename `flyway.flyway` ns to `flyway.core`:

 - Update the constructour function to accept location for migration
   dir
 - Update constructour to work with 3.0 Flyway class

Remove flyway.metadata_dumper: no longer needed

Update leiningen plugin:

 - Unify all commands into single namespace
 - Update commands to work with 3.0
Update project.clj: remove postgresql dep
Update docstring ordering

Update README.md