Spur

* Add support for platforms that don't support the pty module, such as Windows.

* SshShell: Use "which" if "command -v" is not available. Fixes GitHub issue 15:

https://github.com/mwilliamson/spur.py/issues/15

* Treat output as bytes rather than attempting to decode to string when
generating RunProcessError.message. Fixes GitHub issue 13:

https://github.com/mwilliamson/spur.py/pull/13

* Support unicode commands over SSH.

* Add full support for Python 3.

* Handle buffering more consistently across different Python versions.

* LocalShell: Add support for Python 3. Since paramiko is currently unsupported
on Python 3, use the package "spur.local" rather than "spur".