remove option --debug and update documents
Merge pull request #918 from menghan/remove-deprecated

Remove deprecated option --debug, fix #917
Add a news entry for #917.
Use tox in .travis.yml.

This way we can avoid code duplication.

Also, this change makes tests faster due to ignoring the sdist step
and adds Python 3.2, 3.4 and PyPy 3 to build matrix.
Merge pull request #919 from berkerpeksag/cleanup-scripts

Use tox in .travis.yml.
uses gunicorn.selectors when python < 3.4
gthread: improve accepting connections loop

make sure we stop accepting immediately when max connections is achieved.
allows to enable/disable the use of sendfile()

fix #856
fix method call.
Add a versionadded directive to the sendfile documentation.