Correct references to other squirrelphp libraries

The libraries are (at this moment) not available yet, but will follow shortly. We need to make sure they are at least correct.

Add TestHelpers to release

v0.5.2

Make TestHelpers available in releases

Remove tests from release

In v0.5 we still by accident included the /tests directory. This release now only contains the essential package files.

First official release

Classes and interfaces should be stable, but just to be sure we are starting at v0.5 and will see if any changes need to be made until we do a v1.0