New enthusiastic contribution guide based on Audreyr's cookiecutter-pypackage

Audrey's code is BSD, so there should be no problem with licensing.

I've covered:
* contribution types (with hints)
* steps to start with pytest development
* testing pytest
* basics of hg
Moved contribution guide to the rootdir/CONTRIBUTING.txt
Addressed contribution guide issues: virtualenv, links, Github mirror, RST rendering
Contribution guide: moved tox inst. instructions up
xfailing test for captire encoding issues with binary stdio
initial code import for capture transfer
rewrite all testing uses of py.io to _pytest.capture
rewrite all _pytest.capture uses of py.io to _pytest.capture
simplify StdCaptureFD snapshot reading
move imports and declarations to the top
kill the str magic of Encodedfile
small cleanp
kill ancient capture devnullpath, os.devnull exists since py 2.4
capture tests: move imports and declarations to the top
add notes on the copied pylib version
stop exposing capsys/capfd.capture
backing out Ronny's PR because it was merged too early (still has failing tests)
mark encoding test as xfail also on py2
merge pbanaszkiewicz/contributiondocs
add Piotr Banaszkiewicz