Skip instead of fail tests when BIOBLEND_GALAXY_URL and BIOBLEND_GALAXY_API_KEY environment variables are not defined.

Like all other TestGalaxy* files do.
Deprecate create_user() method in favor of clearer create_remote_user(). Add create_local_user().

create_local_user() is also going to be used for Continuous Integration testing.
Don't fail the test if current_history is None.

Useful for Continuos Integration testing where the user is created via the API and does not have a session.
Move wait() method to Dataset superclass, so it can be applied also to LibraryDatasets.