Initial working numerical range and date filters using new filter mechanism.

Commented-out test code is in inv_item:
https://github.com/ptressel/eden/blob/master/modules/eden/inv.py#L699
There are no inv_item records with either purchase or expiry dates set in the IRC_Train data (which I was using for testing), so need to set some dates to for the date filter to find. I don't know if this is a "feature", but if the date isn't set, it doesn't match if a ge or le query is present. That could be another ToDo -- letting the user say include records w/o dates.

It was easier to add one hidden field to hold each URL query term (for each end of the range), rather than unpacking them from the xxx&yyy form in a single field -- wasn't a good way to tell which query term went with which input value.  If you're ok with that for other filters with multiple query terms, we can replace the (new) data_element method with the (also new) data_element_multiple method.

S3DateTimeWidget needed its selector selection changed to use the _id if that was supplied.

There are a bunch of ToDos in comments just inside the class statements.

There may be some opportunities for simplification of the various S3FilterWidget interlocking methods.  ;-)