Implemented composer dependency manager

I took a look in the vendor libraries that TBG is using, and found out that all versions that are used can be installed via the widely adopted Composer Dependency Manager. This will reduce the TBG-install package size, but is will be wise to also provide a package that already has the vendors installed (for the non-cli users). The current libraries now will be installed installed via Packagist.org:

        "mrclay/minify" : "dev-master",
		"easybook/geshi" : "dev-master",
		"michelf/php-markdown" : "1.3",
		"swiftmailer/swiftmailer" : "4.2.*"

This pull request is an initial setup for the v3.2 branch, so maybe it is a great idea to implement this on the v3.3 branch. I would be happy to help to make a merge succesful for the v3.3 branch, but it looks like that is a private one..