Rewritten core

* support stand-alone (chain length: 1) or own CA (chain length > 1) certificates
* use Lombok
* decode Basic Constraints correctly with own ASN.1/DER decoder
* detect whether certificates are already trusted locally
* ignore HTTP errors when fetching the certificate
* try to give more useful information in error messages
* better GUI/core separation
* German translation
* tests
* version bump to 0.9