Issue #3 : Theme support

Heavy internal BC break: adds a package abstraction layer
No usage BC break
updated documentation for theme support
added database migration support
fetch versions/tags from the developer tab of wordpress.org instead of relying on SVN repo.
added replace in composer.json so wpackgist/plugin and wpackagist-plugin/plugin are not installed at the same time
changed provider groups to last-week, last-2-months, this-year, last-year and old
php-cs-fixer
used new version of RollingCurl using clearCompleted()
Reduced memory usage by clearing response text after processing.
Text changes.