FEATURE: HKPS and Machine readable format

Hi- this is my first contribution to a gnome project. I wanted to search some keys from hkp key-servers and I was surprised that no TLS is supported. I found some 10 year old tickets and decided to implement it. On the way I also discovered that seahorse does html parsing, which was partially broken for me. Also replaced it with the machine readable format.

• HKPS Support: partial, only the hkps part #47
• Use the machine readable format to parse hks requests: fixes #135 (closed)
  Spec: https://tools.ietf.org/html/draft-shaw-openpgp-hkp-00#section-5.2

remarks:

• Adding of HKP/HKPS Servers is broken due #262 (closed) - workaround, add hkps server manually via dconf
• Some Keyservers deliver Keys without an User Identity (privacy reasons, ex. keys.openpgp.org).
  • Key is only displayed in Seahorse when also a uid is available due seahorse wants user information.
• HKP Keyserver do not deliver Subkey Information via Machine Readable Format.