Add more people to AUTHORS

From a suggestion of Anders:

> Actually, since it’s probably best to keep them in an order that doesn’t
> change over time, here are the authors sorted by when their first commit
> was merged:

$ git log origin/master --pretty='%an' --topo-order --reverse | awk '{if (!h[$0]) {print $0; h[$0]=1}}'

I removed duplicates and most of the middle initials.
Fix spelling errors in AUTHORS (caught by ispell)
Add tests to detect new_variable_* overriding previously set values

[davidben@mit.edu: Fix some nits]
Make the fullvar variable test more reasonable

Calling set on the output of get really should a no-op. Also test that
the new version of the setters were installed.
Don't g_value_unset an uninitialized GValue

Silences a GObject assertion.
When replacing a variable, keep the old value

This unbreaks variable behavior on module reload.

Reported-By: Robert Jacobs <rnjacobs@mit.edu>