FEATURE: ability to skip site package installation. NEOS_APP_SITE_PACKAGE=false by default.

With this change NEOS_APP_SITE_PACKAGE is not set by default to 'TYPO3.NeosDemoTypo3Org' anymore. By default it is FALSE which means site package will not be installed. In the descendant images you have to specify NEOS_APP_SITE_PACKAGE if you want to install one.
Flow support pre-requisite: files reorganised, 'neos' part removed from all filenames
CircleCI: cosmetic improvements
Cosmetics: split files into few (separate files for variables, functions)
Flow support pre-requisite: more generic variable names, improved tests
FEATURE: Flow support (1st draft)

Warning: this will be new minor version as it contains breaking changes (all ENV variables changed).
Flow support: README update
Merge pull request #2 from million12/dev

Flow support: ability to create both Neos or Flow applications from this Docker image