moving the library to PSR-4

Hi,
I just found your library and I found it really cool so I did some fixes.
- added `PHP 5.6` to travis
- move from fopen to `SplFileObject`
- move from `PSR-0` to `PSR-4`
- made your library `PSR-2` compliant

I also made some little refactoring to enhanced the code coverage.

Tell me what you think..
When deleting a key this check is not necessary, IMHO, it's a micro optimization. the `unset` will never failed whether the `$key` exists or not on the `$data` property.