Tool can be installed as dependency with Composer

Hello,

this PR makes it possible to install changetrack with Composer as an dependency.

- added missing dependencies for "require"
- minimum PHP version is now 5.5
- added a name and some meta data to composer.json
- "src/bin/track" is now able to find "autoload.php" also when installed under "/vendor/qafoo/changetrack"

P.S. I really like the idea behind this tool :)