All scripts in libexec/ (excluding pyenv) are called through pyenv,
therefore the shebang lines are not necessary. On some systems this
provides a measurable increase in performance of the shell prompt.
Related to pyenv/pyenv-virtualenv#259
* verify checksum of downloaded archives.
* add PYTHON_BUILD_MIRROR_URL to use mirror site.
But we don't have CloudFront setup as of now :-(
* rbenv 0.4.x style help messages