Fields.NumberArray and minor tweaks

Added Fields.NumberArray.  Works mostly the same as CharArray.

On choice fields, made filter() and choices() available on the
FieldInstance as functions; also added a _parent attribute to
FieldInstances and ModelInstances so you can reference them for choices
/ filter functions (e.g. using values of parent models and other fields
as choices).

Began preparing client-side upload field to replace the current server
call to knox.  DOES NOT WORK YET.