Added jsencoding (from node-imap) and jconv as alternatives to iconv and...

... iconv-lite for extra decoding options

Hi andris9 -- love your imap libraries. I had some trouble with garbled Japanese emails – they had been encoded in JIS rather than utf8, when sent from iPhones – so I changed this encoding library to first try jsencoding, which has been copied directly from node-imap. It also tries jconv if iconv-lite didn't work.

They've worked for me on the couple trials I've done, hopefully you can make use of them!

I've also edited mailparser.js in mailparser (I'll send a separate pull request), because I had some Japanese emails arrive as "7bit" instead of "quoted-printable".