* Search pyproject.toml in cwd parents.

* Fix error message when pyproject.toml is not found.

* Attempt to fix Python 2.7 and 3.4 compatibility.

Removes duplicated "(multple values allowed)" information from `poetry
help install`:

    % poetry install -h
    Usage:
    …
      -E, --extras=EXTRAS   Extra sets of dependencies to install
      (multiple values allowed). (multiple values allowed)

* Install using Python version running get-poetry.py script.

* Fix Homebrew distutils issue.

* Revert --install-options; breaks building wheels.