Fix for Issue 40 - Expr does not have a betweenProperties(...) method.

src/main/java/com/avaje/ebean/Expr.java

@@ -73,6 +73,14 @@ public class Expr {
     return Ebean.getExpressionFactory().between(propertyName, value1, value2);
   }
 
+  /**
+   * Between - value between two given properties.
+   */
+  public static Expression between(String lowProperty, String highProperty, Object value) {
+    
+    return Ebean.getExpressionFactory().betweenProperties(lowProperty, highProperty, value);
+  }
+  
   /**
    * Greater Than - property greater than the given value.
    */