Improved error message when leaving out url scheme

Replace urlparse.urlparse with urllib3's parse_url to get slightly better
error messages when leaving out the scheme of the url to test. urllib3 is
already part of the requirements as it's bundled with requests. I found
urllib3 to be easier to use - urlparse tended to put the host into the scheme
instead of None. 

Before:
$ boom localhost:8000/

-------- Errors --------
No connection adapters were found for 'localhost://0.0.0.0:80/8000/'


After:
$ boom localhost:8000/

-------- Errors --------
Invalid URL u'//127.0.0.1:8000/': No schema supplied