log(Product(...)) -> Sum(log(...))

https://code.google.com/p/sympy/issues/detail?id=3661&thanks=3661&ts=1361550326

```
In [19]: simplify(log(Product(f(i), (i, 1, n))))
Out[19]: 
   ⎛  n       ⎞
   ⎜┬───┬     ⎟
log⎜│   │ f(i)⎟
   ⎜│   │     ⎟
   ⎝i = 1     ⎠
```
I would like to see this simplified to 
```
In [21]: Sum(log(f(i)), (i, 1, n))
Out[21]: 
  n            
 ___           
 ╲             
  ╲   log(f(i))
  ╱            
 ╱             
 ‾‾‾           
i = 1          
```