Remove custom styles

Finally it comes.

- Move every styles on `custom/_styles.scss` to `base` or `partials`.
- Removed screen size dependent styles if `custom/_styles.scss` overrides it.
- Use default `margin-left` and `margin-right` on `#content div.pagination`.

Before:
![screenshot 2014-07-05 23 29 59](https://cloud.githubusercontent.com/assets/853977/3486787/51334b72-0451-11e4-97ed-0b83a489ebb0.png)

After:
![screenshot 2014-07-05 23 32 24](https://cloud.githubusercontent.com/assets/853977/3486788/55e9b020-0451-11e4-82c0-63845d42a5ec.png)

Look at the page border lines. `#content div.pagination` depends on screen size and main content also use same `margin-left` and `margin-right` with it. So I removed overriding styles.

I think we should change the comments for `custom/_styles.scss`! :smile: