Moved code into src/ subfolder.
Broke code out into different files.

This is part of a refactoring process in preparation of putting sspak into a phar.
Note that 'sspak bundle' is probably broken by this.
Added bin/build-phar to create build/sspak.phar as a bundled script.

sspak can now be distributed as a Phar, much like Composer.