Add unparse_url to urllib3.util.url

This round-trips with parse_url (or at least equivalently by the RFC).
Add url attribute to Url, which gives the same thing as unparse_url()
Improve code coverage
Remove unparse_url, in favor of Url.url
Make something a little clearer
Merge branch 'master' into url_unparse

Conflicts:
	test/test_util.py
Make str(Url) == Url.url
Add myself to CONTRIBUTORS
Merge pull request #394 from asmeurer/url_unparse

Add unparse_url to urllib3.util.url