Decode downloaded contents.

Also add a test (the exact url depends on pulling a change to pip-test-package).
Change test_requirements:test_remote_reqs to use hltbra's gist

g2p's pull request for issue #760 requires changes on pip-test-package.
The gist was created to remove this dependency.
Move python3/python2 details from ``download.get_file_content`` to backwardcompat module
Tweak variable name at backwardcompat.get_http_message_param (python 2 version)
Merge pull request #874 from hltbra/remote-reqs-py3-pull-request

Fix issue #760: broken external requirements on Python 3