Vagrantpy

- Support Python 3 in addition to Python 2
Author: Volodymyr Vitvitskyi (https://github.com/signalpillar)
- Use `os.devnull` for Windows compatability.
Author: Renat Zaripov (https://github.com/rrzaripov)

- Fix regression where vagrant commands were being printed to stdout.
Author: Todd DeLuca (https://github.com/todddeluca)

- Allow redirection of the output of the vagrant command subprocess to a file.

In order to log the output of the subprocess that runs vagrant commands,
or alternatively to silence that output, `Vagrant.__init__` accepts two
parameters, `out_cm` and `err_cm` that are no-argument functions that, when
executed, return a context manager that yields a filehandle, etc., suitable
for use with the `stdout` and `stderr` parameters of `subprocess.Popen`.
Author: Manuel Sanchez (https://github.com/omoman)
Author: Todd DeLuca (https://github.com/todddeluca)

- add instance attribute `Vagrant.env` which is a mapping of environment
variables to be passed to the vagrant subprocess when invoked. This allows
basic inter-process communication between Python and Vagrant via environment
variables.
Author: Alex Conrad (https://github.com/aconrad)

- `Vagrant.__init__` now accepts a keyword argument `env=None` which will be
assigned to the instance attribute `Vagrant.env`.
Author: Alex Conrad (https://github.com/aconrad)

Oops. Pushed non-master branch to PyPI, for version 0.5.4. Pushing master
branch for 0.5.5.

The major change in this version is switching to using `--machine-readable` in
some vagrant commands to make the underlying `vagrant` commands return