[Featureserver] Equivalent of fileter encoding

[Yves Moisan]

> http://MyHost:8080/MyFSLayer?queryable=MyField&MyField__gte=ABCD-0135&MyField__lte=ABCD-0184 : NO FILTERING

The select method in postgis.py does the right thing by putting "AND" by
default in the list of filters.  The problem lies in the
feature_predicates function which cannot take more than one pair of key
values for the same key.  

The list of predicates returned for the above query is 

['MyField >= %(MyField)s']

for
http://MyHost:8080/MyFSLayer?queryable=MyField&MyField__gte=ABCD-0135&MyField2=ABCD-0184

['MyField2 = %(MyField2)s', 'MyField <= %(MyField)s']

I'm not sure how to go about this, but I know it means probably not
using the zip() function.  If that is deemed important enough an issue
(date <= && date >= anyone ?), then that's where the problem is in the
code.

Yves, trying to lower his consumptive/generative ratio ...