Add Lead function which is similar to Oracle's same name window function

Lead is an analytic function like Oracle's Lead function. It provides access to more than one tuple of a bag at the same time without a self join. Given a bag of tuple returned from a query,  LEAD provides access to a tuple at a  given physical offset beyond that position. Generates pairs of all items in a bag.

If you do not specify offset, then its default is 1. Null is returned if the offset goes beyond the scope of the bag.


Example 1: 
```
   register ba-pig-0.1.jar

   define Lead datafu.pig.bags.Lead('2');
   
   -- INPUT: ({(1),(2),(3),(4)})
   data = LOAD 'input' AS (data: bag {T: tuple(v:INT)});
   describe data;

   -- OUTPUT:  ({((1),(2),(3)),((2),(3),(4)),((3),(4),),((4),,)})
   -- OUTPUT SCHEMA: data2: {lead_data: {(elem0: (v: int),elem1: (v: int),elem2: (v: int))}}
   data2 = FOREACH data GENERATE Lead(data);
   describe data2;
   DUMP data2;
```

Example 2
```
   register  ba-pig-0.1.jar

   define Lead datafu.pig.bags.Lead();
   
   -- INPUT: ({(10,{(1),(2),(3)}),(20,{(4),(5),(6)}),(30,{(7),(8)}),(40,{(9),(10),(11)}),(50,{(12),(13),(14),(15)})})
   data = LOAD 'input' AS (data: bag {T: tuple(v1:INT,B: bag{T: tuple(v2:INT)})});
   --describe data;
   
   -- OUPUT: ({((10,{(1),(2),(3)}),(20,{(4),(5),(6)})),((20,{(4),(5),(6)}),(30,{(7),(8)})),((30,{(7),(8)}),(40,{(9),(10),(11)})),((40,{(9),(10),(11)}),(50,{(12),(13),(14),(15)})),((50,{(12),(13),(14),(15)}),)})
   data2 = FOREACH data GENERATE Lead(data);
   --describe data2;
   DUMP data2;
```