Fix #833 by limiting Windows shell_env support to local() only.

Also fixes changelog to note that the original #804 was considered a bugfix, not a feature, since it is in the 1.5 branch
Merge branch '1.5'
Add rtunnel() context manager to open a reverse tunnel (from server to client)
Fix coding convention
Change to 4-arguments API, and unnest internal functions.
Update documentation
Rename remote_host into remote_bind_address
Nest _accept back into remote_tunnel, and generalize _forwarder.
Remove unused import.
Changelog + docstring updates re #821
Fixes #587 - warn on missing ssh config, don't abort
Hide some test output
Merge branch '1.5'

Conflicts:
	docs/changelog.rst
Try adding CONTRIBUTING doc w/ link to real docs
Merge branch '1.5'
meh
Merge branch '1.5'
Maybe this?
Use Sphinx include for CONTRIBUTING
Derp