Modified for Singleton instance

Changed the class to work as a Singleton instead of using a global
variable to store the instance of the class.  Also cleaned up the
spacing to make it prettier.
Update php-hooks.php
fixed typo
[+]: added some debug-code
[+]: use as a singleton
[+]: fixed syntax & phpdoc
[+]: added first simple PHPUnit Test (more work needed)
[-]: removed old file
[+]: added "travis-ci"-config
Update composer.json
fixed license - http://spdx.org/licenses/
fixed #6
"Update the inline docs for `add_shortcode()`"

sync with WordPress -> https://github.com/WordPress/WordPress/commit/947aa049003582acca152001ad0de39877321e3e
"Make `has_shortcode()` recursive/work for nested shortcodes."

sync with WordPress -> https://github.com/WordPress/WordPress/commit/1292ee01519304c939c3528ed211bef5e1e55186
"After [29197], use `! empty()` instead of `isset()`."

sync with WordPress -> https://github.com/WordPress/WordPress/commit/3ca4223a00e4d52e873581e7b7fd8ba786a1f5d4
[!]: fixed "has_shortcode()"
Merge branch 'master' of https://github.com/voku/php-hooks
Shortcodes implementation moved in separate class file