[#8] envdir.write: python 3 FileExistsError/OSError fixes

On python 3 ``os.makedirs`` raises FileExistsError not OSError
when given path directory already exists.

README and comments fixed as well.
[#8] envdir.write: no new line appending
[#8] envdir.write: default envdir path calculation extracted to separate method
[#8] envdir.write: path param is now optional
[#8] envdir.write: write method signature changed

Referring the discussion: https://github.com/jezdez/envdir/pull/9

This changes allows to use ``envdir.write`` in two follwing ways::

    # 1) with default path
    envdir.write({'VAR': 'value'})

    # 2) with the path provided by user
    envdir.write('/home/jezdez/mysite/envs/prod', {'VAR': 'value'})

Provided solution leaves room for additional kwargs arguments if we
will later want/need them.