Add gpg stanza to compatible casks

Add `gpg` stanza to casks for which keys and detached signatures are available. Feel free to add entries to the list.

Known compatible:  
- [x] cryptol
- [x] electrum
- [x] electrum-ltc
- [x] espionage
- [x] git-annex
- [x] gpgtools
- [x] horndis
- [x] jameica
- [x] libreoffice
- [x] linphone
- [x] litecoin
- [x] lyx
- [x] macports
- [x] multibit
- [x] mumble
- [x] mysqlworkbench
- [x] onionshare
- [x] pgadmin3
- [x] ricochet
- [x] torbrowser
- [x] vlc
- ~~armory~~ — no detached signature
- ~~bitcoin~~ — no detached signature
- ~~wireshark~~ — no detached signature