Merge pull request #1517 from astrofrog/minor-warnings-fix

Minor warnings fix
Merge pull request #1526 from astrofrog/nddata-stats-time-six

Refactor astropy.nddata, astropy.stats, and astropy.time to use six instead of 2to3
Control whether metadata merge conflicts result in a warning, an error, or nothing (only in vstack for now)
Improved description for ``metadata_conflicts`` and add the option to ``hstack`` and ``join``.
Ensure that an exception is raised if metadata_conflict is not one of e expected values.
Fixed typo