keepass2 file support

Hello, these changes will allow python-keepass-http to also serve from keepass2 files.

There are however some things to consider:
First, I am not really familiar with your dependency management system, so I added the required libkeepass manually to the project - you might want to fix that (or even better, tell me how to do it myself ;)
Second, the libkeepass version that I included contains a minor change from me in comparison to the original lib (because the original lib did not write the HeaderHash property, which is mandatory to be correct for newer keepass clients) - you can find my updated version here https://github.com/phpwutz/libkeepass

Finally, I had some issues starting the server at all when the serve_forever() method was called inside the daemon_context block - forking did work apparently, but the server did not send an answer to requests. Maybe I'm just missing something here that could be easily explained.