Minor update to tests, added docstrings

Requested by @sigmavirus24.
Merge pull request #1341 from cdunklau/docstring_test_1329

Minor update to tests, added docstrings
Fixed example in authentication docs
Issue #749: Add optional SNI support for python2.

SNI support will be enabled for python2 if ndg-httpsclient and pyopenssl are available.
Merge pull request #1347 from hobarrera/master

Issue #749: Add optional SNI support for python2.
Adding link to documentation in README.rst

Just adding a link to the documentation at python-requests.org
Merge pull request #1342 from mexicat/master

Fixed example in authentication docs
Merge pull request #1352 from nicoddemus/docs-link-on-readme

Adding link to documentation in README.rst
Prevent a KeyError when there's no link headers
Merge pull request #1355 from sigmavirus24/fix_links

Prevent a KeyError when there's no link headers
Fixes #1320: transport adapters stored in ordered form