Implement subprocess.run()

Make it available to all Python versions, not just 3.5. It is
cooperative on all versions.

This requires some minor updates to the Popen API (being a context
manager and having the args attribute on Python 2) and works best when
the TimeoutExpired exception is also defined in Python 2 (as a subclass
of Timeout, for BWC).

This prompts me to wonder if some of the duplicated Py2/Py3
methods (like check_call) can be combined now. I think I'm generally OK
with API extensions so long as there are no BWC concerns, and the
context-management, in particular, is nice for those methods.

Fixes #872.

* Fix #878

We can use multiple watchers to avoid any race conditions.

* Specific test cases for lines in os.waitpid.

* Less strict on the test since we may be running in parallel.

* make the leakchecker happy.

Support for and test Python 3.6

- Support encoding/errors args.
- Decode env items if necessary.
- Flush stdin during communicate.

Enable this test again on python 3 on travis.

It appears the gevent.signal module is not the source of the problem.

Also stop wrapping methods over and over. Yikes.

Also install 2.7.12 on travis.

Also update the SSL test for Python 2.7 to test the fix for Python bug
 no. 23804. This may need some tweaking for the 2.7.8 environment to
disable some tests. We'll let Travis run 2.7.8 to find out.

Make manylinux fixes

Allow resetting the C state for whether libev child handlers are active.

Use this when we disable them in gevent.signal so that next time we use
gevent.subprocess it will work.

Fixes #857.

Fixes #856

[skip ci]

Document that kill() is not always safe for greenlets that aren't generally exception safe. Link to article about same. Ref #846.

[skip ci]

[skip appveyor]

Travis and Appveyor are both green.