Silence stderr for reader tests

The reader tests were relying on sys.stderr being mocked prior to their runs by
other tests, which is why a rename triggered this error. Problem was solved by
wrapping them in MockedGlobals subclasses.
Moved salve.settings.config to salve.config

Rather than having a whole package devoted to settings, which does not properly
encapsulate the global execution context and other classifiable "settings", the
config package is exposed within the toplevel salve package. This is a clearer
breakdown. Config can be turned into a package if there is a call to do so in
the future.