Start work on sending cookies back

On 401's the cookies received aren't sent back to the server. See: #1336
Make the regular tests pass

I broke Digest Auth completely
Fixed finally. Also requires updated httpbin.org
Take advantage of the new copy method
Proxy urls should have explicit schemes.
Header keys should be native strings.

This commit follows a discussion on IRC. For more information, see the
Pull Request associated with it.
Modify new CID tests to use native string keys.
Instantiate the CID directly.

No need to do this the slow way now. Thanks to Colin (@cdunklau) for the
idea.
Fixing error return to be consistent with tuple return at the end of the method.
Making sure we check for str/unicode.
Merge branch 'keytype' of git://github.com/Lukasa/requests into 2.0
Merge branch 'proxyscheme' of git://github.com/Lukasa/requests into 2.0
Merge branch 'fix_401_cookies' of git://github.com/sigmavirus24/requests into 2.0
Better percent-escape exception.
Merge pull request #1514 from Lukasa/verror

Better percent-escape exception.
Update urllib3 to 14b8945
Fix for new proxy API.
Cache manager for proxy connection.
Added proxy support to feature list.
Added entry to AUTHORS.