Adding logging messages and default values check

Hello:

I've made some more changes:

* Fix setting name ```keep_pdf_files``` into ```KEEP_PDF_FILES``` since Django was not loading the value properly.
* If the ```basename``` is indicated and ```KEEP_PDF_FILES``` is ```True```, the file is written with the indicated name, not just with the UUID string generated. I've made it since I need to reach the saved file with a known name, not only from inside the view that generates the file.
* All default values for constructor are ```None```: if some value must be setted, then it must be defined on ```DEFAULT_SETTINGS```.

Please, let me know what you think about this changes.
Regards.