Add support for `fragment` in `parse_url` and `build_url`

Any opposition to adding support for the [fragment](http://tools.ietf.org/html/rfc1808.html#section-2.4.1)? It looks like `parse_url` already [grabs it](https://github.com/hadley/httr/blob/a9492849d44198c53148b1d8681bb2869c51b0e8/R/url.r#L43).