Small improvements for pygpghttpd

[krautsource]

Hi!

1. Shebang: I'm completely new to python, but it seems like "python" defaults to python2 on most systems. Hope it's okay to make it say "python3".
2. Query string: I was wondering at the workaround for "+"-encoded blankspaces which didn't work for me, so I replaced the whole query string parser with the one included in urllib.parse -- which also handles the blankspace encoding along the way.
3. example.htm: When listing all the keys, only the last one was displayed.

[krautsource]

Did some more work on pygpghttpd... using this as an opportunity to get more familiar with Python :-)

• The original socket handling routines couldn't cope with larger payloads where not all of the data was available in the socket buffer right away. In my case (Chromium on Linux), this happened consistently with request bodies larger than 856 bytes -- the server would always tell me that no cmdstr was received.
  So I took the liberty to employ the http.server.HTTPServer module for all HTTP and connection handling. This also somewhat reduces code and complexity.
• The file accepted_domains.txt was read on every request even though the file is unlikely to change between requests. I changed this to execute at startup instead.
• Signatures were completely ignored when decrypting messages; AFAICT, python-gnupg doesn't tell the caller whether signature verification succeeded or not. So I resorted to checking for "ERRSIG" in GnuPG's stderr output after the decrypt() call. If you can think of a better way, please let me know.
• I also added some error handling though this could still be refined by parsing GnuPG's stderr output for markers like "BAD_PASSPHRASE" etc. to create more meaningful error messages.
  Not sure if that's a direction you'd like to go, so I'm waiting for your feedback on this.

I hope these changes are okay with you.

One more thought on returning data from pygpghttpd to the client:
IMHO a JSON struct should be returned to the client even for encryption/decryption/signing/verification etc. with three fields like "statuscode", "message" and "result", so the client can tell error messages from GnuPG results and display both of them accordingly.
One example would be an encrypted & signed message that decrypts ok but fails signature verification. In that case, the client probably should display the decrypted message plus an error message regarding the signature. Currently that's not implemented with pygpghttpd.
What do you think?