Fix spacing in strip_html for paragraph and header elements

cc @csaunders @fw42 

Adds spacing to the strip_html filter for paragraph and header elements. 

![image](https://f.cloud.github.com/assets/403774/2513016/3488f46a-b42c-11e3-8a95-96736e433ef3.png)

Presently if you use the filter on something with header or paragraph elements (like search results on a shop), it doesn't add any spacing between the two elements where there would usually be a line break. This adds a space in between closing and opening tags of this type would be.

Let me know if there's any other elements like this that should behave the same way, possibly divs/spans as well?

(sorry for premature notification, fat-handed the keyboard)