Work on getting conda to prompt for proxy username and password

It doesn't seem to work for https proxies.
Add missing raise_for_status() in download()
Add unparse_url from https://github.com/shazow/urllib3/pull/394
Include the original exception with the ValueError

This is in case the error comes from something else that we didn't expect it
to.
Get default proxies if none are set in .condarc in conda.config.get_proxy_servers()
Prompt for proxy username and password

Thanks a ton to @Lukasa for help with this.
Merge branch 'master' into proxy-pass

Conflicts:
	conda/fetch.py
Merge pull request #739 from asmeurer/proxy-pass

Work on getting conda to prompt for proxy username and password
Merge branch 'master' into rpath