3341 - allow powsimp to process coefficients

https://code.google.com/p/sympy/issues/detail?id=3341

master processes exact coefficients

```
>>> powsimp(2*2**n)
2**(n + 1)
```

in this PR factors can be removed from coefficients

```
>>> powsimp(4*2**n)
2**(n + 2)
>>> powsimp(16*2**m*8**n)
2**(m + 3*n + 4)
```