Merge pull request #38 from mdboom/update-for-new-vounit

Fixes for newer versions of astropy and numpy
Update adsf-standard
Merge pull request #37 from mdboom/custom-tags

Make it easier to define custom types.
Make it easier to define custom types.

Change how the mapping from tag names to schema urls, and schema urls to
local files works.
Make byteorder explicit