added hr styling to post page

Hello Shashank

Loving your theme, I've shown it to a few friends of mine and they went nuts!

Just another small change I've made to my copy of the theme and I think it would be useful for everyone else.

The `hr` is unstyled, noticed when using it via markdown (`---` on a blank line) and I've decided to fix by applying the same colors as the top/bottom borders of `article` and `div.share`, for example, to preserve the look 'n' feel.

Here's how it was before the fix:

Before:
![image](https://f.cloud.github.com/assets/1674699/486388/fdf208fe-b91f-11e2-9fec-eb16909e84ca.png)

After:
![image](https://f.cloud.github.com/assets/1674699/486403/e09c716c-b920-11e2-9f2c-b0f15f2ec73c.png)

Hope you like it

Thanks again!