modify *

There was an issue with additional 0s being added at the end. For example, this was causing an error


```
using Polynomials
x = poly([0.0])
polyder(x^7)
```

as (x^7).a had 26 terms.

This might be better addressed with just `n=length(p1) - 1`.
Merge pull request #6 from jverzani/patch-2

modify *