ACE field: dropped sync call to "ace.require" (#78)

The ACE field was using a synchronous call to "ace.require" to
initialize the mode but that call only succeeds provided the mode
has already been loaded.

Using "setMode" does the exact same thing if the mode has already
been loaded and attempts to load the mode in the background otherwise.
The minItems/maxItems properties of a nested array were ignored (#79)

If the form referenced a schema key that contained nested array keys,
the "minItems" property of these nested array keys was simply ignored.
Moreover if the first array level contained a "minItems" property,
it was incorrectly applied to the nested array keys as well. Same
thing for "maxItems".

See issue #79 on Github for a concrete example.

The schema key reference was not properly processed. Two cases arise:
1. the form element references a schema key. That schema key is the
one to look for minItems/maxItems properties.
2. the form element does not reference a schema key, but some of its
children do. The closest parent array of the referenced schema keys
are the ones that may define minItems/maxItems properties.