Min-height should be specified for posts to fix issue with border

The following problem occurs when using this theme:
![issue](https://f.cloud.github.com/assets/646942/1713161/fa55f8d2-6184-11e3-9c3a-f460d8971adb.png)

Basically the body of the post is too short and the border does not reach the bottom line. We should specify minimum height for post and for its content. Result:
![fix](https://f.cloud.github.com/assets/646942/1713171/23c2b156-6185-11e3-9e49-8c0a8178e715.png)