Improves on #876: Doesn't define `as` for `arcsecond`, only `mas` and `uas`.  Also makes up for the fact that we don't have a unit called `as` (which used to correspond `attosecond`) anymore, since it's a Python keyword and is quite confusing, though `attosecond` still exists for those crazy people that might need it.
Add __str__ and __repr__ methods to config item
Update config item docs and minor code cleanup
Fix #861: Fix values of u.a, u.yr and u.lyr
#865: Provide a way to hide the download progress.
Fix definition of lightyear in test
Remove extra space in section heading
Merge pull request #855 from taldcroft/config/str-value

Improve usability of ConfigurationItem with better repr and str methods
Merge pull request #861 from mdboom/unit/lightyear

units: u.lightyear and u.yr
Merge pull request #892 from mdboom/unit/simplify-arcsecond

Units: Improve arcsecond definitions
Merge pull request #865 from mdboom/console/hide-download-progress

Suppress "Downloading" message in download_file()