Clean up JSLint errors in mailpile.js

This PR cleans up a number of JSLint errors in mailpile.js, as well as standardizes mixed tabs and whitespaces to whitespace and a 4-space indent.  I've not gotten Mailpile running locally yet and don't have access to a broad spectrum of browsers, so I haven't been able to test very well.  I don't know if you're interested in this sort of thing or not, but I thought I'd take a swing at it and see if it started a conversation or got shut down.