I have have a dataframe that is a record of deliveries. How can I create a summary dataframe that has months in columns, customer # in rows and sum of quantity per month?

```
> my.data
   yr   mo    cust    qty
1  2010  6 2005161 620448
2  2009  5   29290 533000
3  2011  6   51800 125000
4  2011  7 2005149 677856
5  2009  5   51888 128000
6  2011  1  290913  51400
7  2010  9 2075516 352800
8  2011 11   34029 574200
9  2009 11   40740 734740
10 2011 10 2016135 300209
11 2011  5 2000294 605300
12 2010  3   34004 675200
13 2011  2 2030823 610236
14 2011 12 2016188 682000
15 2010  7   23409  85000
16 2011  1 2089839 188000
17 2011 11   34414 455658
18 2011  7 2004149 166200
19 2011 11   40404 235000
20 2009  7  203203 531000

```