Rewrite of harmonic numbers into summation

Rewrite of harmonic numbers into symbolic sums:

```
In [4]: harmonic(n)
Out[4]: harmonic(n)

In [5]: _.rewrite(Sum)
Out[5]: Sum(1/_k, (_k, 1, n))

In [6]: pprint(_)
  n    
 ____  
 ╲     
  ╲   1
   ╲  ─
   ╱  k
  ╱    
 ╱     
 ‾‾‾‾  
k = 1  

In [7]: _.subs(n, 10)
Out[7]: Sum(1/_k, (_k, 1, 10))

In [8]: _.doit()
Out[8]: 7381/2520
```