added ajax choice list retreival for editing report search criteria
fixed path_verbose for case = 1-M
apply all filters as a single filter (rather than a chain) to avoid unnecessary or problematic joins
fixed filtering on None values
added basic grouping support
fixed sorting for reports with grouping
added post query filtering for choice list display
added string-to-datetime conversion for DateTimeFields
fixed datetime filters for case USE_TZ = True
Merge pull request #15 from SacNaturalFoods/master

completed the select box feature for search criteria with choice fields