Smtp imap support

[johanneshahn]

New transaction method via mail.

The sender can create a new transaction and send it to receiver via smtp.
./epic-wallet send -m "mail" 20 -d receiver@domain.com

The new argument for send method is "mail".

The receiver has to start a listener to process the transactions.
./epic-wallet listen -m imap
The listener will check every minute for unread messages in the mailbox "EPICCASH"
Once the listener finds an unread message in the given mailbox, it imports the tx file from attachment
and sends back the signed transaction to sender.

The sender has to start a listener to process the transactions from receiver to finalize the transaction.

There is a new config required in epic-wallet.toml

#########################################
### IMAP CONFIGURATION ###
#########################################
[imap]
server = "IMAP SERVER"
username = "IMAP USERNAME"
password = "IMAP PASSWORD"
port = 993
inbox = "INBOX.EPICCASH"
reply_subject = "Reply subject"
reply_body = "Move this mail into the epiccash mailbox and mark as unread,<br/>to finalize the transaction. <h1>HTML-Mail Template</h1>"

#########################################
### SMTP CONFIGURATION ###
#########################################
[smtp]
server = "SMTP SERVER"
username = "SMTP USERNAME"
password = "SMTP PASSWORD"
from_address = "YOUR MAIL"
subject = "New epiccash transaction"
body = "Move this mail into the epiccash mailbox and mark as unread,<br/>to process this transaction. <h1>HTML-Mail Template</h1>"