For-beginner-piano

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- Added an alias for 'newbuildout', you can now start a buildout with the
'piano compose mysite' syntax. Updated the README accordingly

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- fixed a bug where alternative plone installs were not cleaning up after
themeselves properly

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- added support for installation of alternative plone versions by setting
special env variables

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- updated the README
- looks for unified installer in the ~/.buildout cache first.
does not re-download unified installer

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- fixed flake8 issues

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- made adjustments to README