add system-wide IPython config

Mainly for use in multi-user administrated environments

- config files in `/usr[/local]/share/ipython` will be loaded
- change priority config loading to load all found files in priority order
  instead of loading only the first found.
`reversed()` on a list gives a `list_reverseiterator` object, so I guess it does avoid making a copy. I think reversed() is slightly clearer than `[::-1]`, but it's not worth changing.