[Fix #689] Use GPG_AGENT_INFO instead of GPG_AGENT_PID

GPG_AGENT_PID may not be defined. Also, source gpg-agent.env before
checking the PID.
[Fix #648] Do not instruct users to call editor-info
[Fix #626] Escape '%' in path
Display full command line on suggestion for the kill command

For the zsh suggestion for the 'kill' command, show full command line
(command: usually with arguments) instead of only the executable name (comm).
[Fix #635] Use $BROWSER to open node documentation
Merge branch 'master' of github.com:sorin-ionescu/prezto