[MRG+1] Use best practices for TLS connections when using Twisted>=14.0

[MRG+1] Change default S3 ACL to "private" and allow customization via settings

Fix documentation for S3_STORE_ACL (now settings.FILES_STORE_S3_ACL) settings: it has nothing to do with feed exporters.

[MRG+1] Add "Content-Length: 0" for body-less HTTP/1.1 POST requests

Revert "Cleanup http11 tunneling connection after #1678"

[MRG+1] Better tracebacks

Fix SkipTest() message for botocore import test

It prints the same as passing `str(e)`

GH-823 was fixed only for HTTP/1.0 (in GH-1089)

Revert test server certificate change

New DOWNLOADER_CLIENT_TLS_METHOD setting to configure TLS method

Fix typos in docstrings

Also remove TLSv1.1 and TLSv1.2 method: these are available only
from pyOpenSSL 0.14
https://github.com/pyca/pyopenssl/releases/tag/v0.14a1

[MRG+1] Py3 S3 botocore

Trying to force Travis CI to build

More liberal Content-Disposition header parsing

Fixes #1782