refactor client models to use lokijs

shared/utils/date_range.js

@@ -1,4 +1,4 @@
-class DateUtil {
+class DateRange {
 
   static addRange(new_range, ranges){
     var gaps_filled = [], new_ranges = [],
@@ -49,14 +49,6 @@ class DateUtil {
     return { gaps_filled: gaps_filled, new_ranges: new_ranges }
   }
 
-  static min(date1, date2){
-
-  }
-
-  static max(date1, date2){
-
-  }
-
   static gte(date1, date2){
     return (date1 === undefined || (date2 !== undefined && date1 >= date2));
   }