Email date and attachments

Notes:
* Date is now microseconds since the epoch
* Attachments are always encoded to base64 in sandstorm-http-bridge.c++
* Included code for base64 encoding from libb64 (http://libb64.sourceforge.net/). Only parts I added were calculating the size of the output buffer and handling the edge case for when there's one less new line than usual
* Had to re-write HackSessionContext.send to use simplesmtp directly for attachments
* Mailpile doesn't support outgoing attachments yet, but I've tested it with incoming attachments