Fixes improper parsing for URLs that span multiple chunks.

See https://github.com/benoitc/http-parser/issues/47.
The issue arises because the on_url callback defined in parser.pyx
overwrites url each time it is called. The http_parser c API calls it
for each chunk of the URL, so the proper way is to append instead of
overwriting.
Merge pull request #48 from ngo/master

Fixes improper parsing for URLs that span multiple chunks.