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.

Unify lots of implementation details between FileObjectPosix/Thread, including making *bufsize* of 0 be respected.