Public API modifications

We had to set the error correction level and QR's version without using the `$_GET`.
Here what we modified to fit our needs. Hopefully, someone else will benefit of it ;)

* Added setter/getter functions to replace `$_GET`.
* Added constants for error corrections level and image types.
* Replaced trigger_error by exceptions.
* Fixed a mistype of `$qrcode_structureqppend_m`.
* Added some PHPDoc.

Thanks.

*P.S. In the branch "code-cleanup", we made the extra-step of "cleaning" the code a little more... Take a look if interested... on the other hand, it's 900+ lines changed...*