Make sure session and default headers play nice

Before: headers = default + args + session
Now:    headers = default + session + args

Fixes #180
Update CONTRIBUTING.rst
Update CONTRIBUTING.rst
Enable testing on Python 3.4
Merge pull request #219 from frewsxcv/patch-1

Enable testing on Python 3.4
Converted built-in formatters to formatter plugins.

Still work in progress and the API should be considered private for now.
Update CONTRIBUTING.rst
Update CONTRIBUTING.rst
Enable testing on Python 3.4
Merge branch 'master' of github.com:jkbr/httpie
Add supported, relevant Python version classifers
Added OSX to Travis CI config.
Merge pull request #220 from frewsxcv/patch-1

Add supported, relevant Python version classifers
Fixed --timeout

* Require requests >= 2.3.0
* Updated test_timeout_exit_status

Close #185.
Add info about SNI on Python 2.x to README

This updates the HTTPS section of the README with instructions on how to get SNI working on Python 2.x.
Merge pull request #222 from felixbuenemann/patch-1

Add info about SNI on Python 2.x to README
Fixed tests.
Merge upstream changes