- move to 0.3 as we are changing API

- move to 0.3 as we are changing API
- [general] The focus of 0.3 is to clean up
  and more fully document the public API of Alembic,
  including better accessors on the MigrationContext
  and ScriptDirectory objects.  Methods that are
  not considered to be public on these objects have
  been underscored, and methods which should be public
  have been cleaned up and documented, including:

    MigrationContext.get_current_revision()
    ScriptDirectory.iterate_revisions()
    ScriptDirectory.get_current_head()
    ScriptDirectory.get_heads()
    ScriptDirectory.get_base()
    ScriptDirectory.generate_revision()

- [feature] Added a bit of autogenerate to the
  public API in the form of the function
  alembic.autogenerate.compare_metadata.