Composer and packagist support

[tikaszvince]

For modern PHP frameworks composer support is essential for proper library and dependency management. I'd like to propose that the library adopts this based on this PR.

Changes contained:

• Define Barion namespace
• Split enumeration.php into smaller chunks, abstract Enumeration classes converted into interfaces
• Update documentations
• Updated the changelog to reflect the composer related updates. Probably this is not a patch or minor release, so please, change the version number

After merge please register this package as Composer package https://packagist.org/packages/submit