Support for Composer 2 & Require PHP 7.1

• Add LICENSE and CHANGELOG.md files
• Add phpunit/phpunit and squizlabs/php_codesniffer as development
  dependencies
• Add requirement for PHP 7.1
• Add support for Composer 2
• Move OomphInc\ComposerInstallersExtender\Installer to
  OomphInc\ComposerInstallersExtender\Installers\Installer
• Move OomphInc\ComposerInstallersExtender\InstallerHelper to
  OomphInc\ComposerInstallersExtender\Installers\CustomInstaller
• Implement PSR-2 standards and PHP 7.1 syntax
• Update project README.md file

Minor syntax update to provide compatibility with PHP 5.3

This swaps out the 5.4 array syntax with the older style to provide compatibility with PHP 5.3. Though we do not intend to actively support 5.3 with future development, we find it reasonable to make this minor change, especially considering composer itself requires a minimum of 5.3.2.

Update composer/installers version requirement

To accommodate the latest release. Thanks @webflo!

Update package requirements to be less restrictive

Thanks @cfoellmann !

Initial release

v1.0

Merge branch 'master' of github.com:oomphinc/composer-installers-exte…