FIX w3lib.http.basic_auth_header should return bytes
Merge pull request #28 from scrapy/bytes-basic_auth_header

FIX w3lib.http.basic_auth_header should return bytes
Bump version: 1.8.0 → 1.8.1
url-related functions accept bytes and unicode and return bytes:

* safe_url_string
* safe_download_url
* is_url
* get_base_url
* get_meta_refresh
urlencode changes the return type differently on py2 and py3
add parse_qs and parse_qsl functions that work correctly with bytes and str in both python versions
use type aware parse_qs(l) on w3lib functions