Remove margin-bottom on nested lists

Nested lists should not have margin-bottom, it breaks the flows of the outer list content.
This is also aligned with how GitHub shows nested lists.

Possible to move the code at the beginning of the line to it's original declaration at line 79 as well:
```
ul,ol{margin-bottom:15px}
```
Before:
![before](https://cloud.githubusercontent.com/assets/480469/3659727/716182f8-11b0-11e4-8713-6ddbbf2af6e6.PNG)

After:
![after](https://cloud.githubusercontent.com/assets/480469/3659732/7b9f74a0-11b0-11e4-9680-3bc5d69874bc.PNG)

GitHub:
![github](https://cloud.githubusercontent.com/assets/480469/3659735/83b0d800-11b0-11e4-8dad-9dd85a7a1fec.PNG)