Fix mixed up worker signal handling

Commit 81241907ffcf94517ffa14b8427205906b61b540 changed the signal
handling by switching the roles of `TERM` and `QUIT` for the arbiter
so that `TERM` is graceful and `QUIT` is not.

At the time, workers performed graceful shutdown on `QUIT` and quick
shutdown on `TERM` and `INT`. This behavior was also changed so that
`QUIT` (and `INT`) cause a quick shutdown and `TERM` is graceful.

However, the documentation incorrectly reversed the roles of the worker
signals and the arbiter was not updated to use the correct signals.

This commit fixes the documentation and the arbiter signals.
Remove unused imports
Merge pull request #746 from ifduyue/remove_unused_imports

Remove unused imports
Stop all servers
Merge pull request #747 from wooparadog/stop-all-servers

Stop all servers
Merge pull request #745 from tilgovi/worker-signals-mixup

Fix mixed up worker signal handling
Remove __future__.with_statement imports.

Gunicorn requires Python 2.6 or newer now:

http://docs.gunicorn.org/en/latest/install.html#requirements
Remove the redundant if clause in gunicorn/workers/workertmp.py.
Add Python 3.4 to classifiers.

I tested Gunicorn with the following command on Pytohn 3.4.0:

    $ python3.4 -m venv venv34
    $ . venv34/bin/activate
    $ pip install -e .
    $ pip install -r requirements_dev.txt
    $ py.test tests/
Also, enable Python 3.4 on Travis CI.

(The "--use-mirrors" option was deprecated, so I removed it.)
Merge pull request #751 from berkerpeksag/classifiers-python3.4

Add Python 3.4 to classifiers.
Merge pull request #749 from berkerpeksag/remove-with_statement

Remove __future__.with_statement imports.
Merge pull request #750 from berkerpeksag/startswith-idiom

Remove the redundant if clause in gunicorn/workers/workertmp.py.