minor adjustments: removed iconv dependency and fixed close method

hi andris,

thanks for the awesome work on the email libs!

for better compatibility with [browserify](http://browserify.org/), i swapped iconv with iconv-lite. this is only a minor issue and iconv-lite should get the job done, too.

close() appears to be buggy, as it calls itself recursively after "LOGOUT" instead of _close(). did i overlook something? we saw this issue as we put inbox on top of an adapter from net.Socket to chrome.socket. if i understood the problem correctly, the suggested patch should fix this.

best regards
felix