Fix remote requirements in Python 3

[g2p]

This was issue #760.
Also add a test (the exact url depends on pulling pypa/pip-test-package#2).

[hltbra]

Unfortunately this pull request works only for python 3. Python 3 http.client.HTTPMessage inherits from email.message.Message, which contains the get_param method.

While on python 2 httplib.Message inherits from mimetools.Message which contains `getparam``. Method name and signature are incompatible.

[g2p]

Sorry, I've updated it to handle both cases.

[hltbra]

Thanks @g2p.

I don't know if it is better to try/catch on `AttributeErroror let that explicit atbackwardscompat/init.py`` and import from there.

Anyway, I will test it and change it later if needed.

[hltbra]

I am closing this pull request and opened #874 with some diffs. Thanks @g2p.