Add TLS support

[joeyadams]

I'm just creating this ticket to prevent duplicated work. I'm working on TLS support right now.

First, I'm going to make a pure interface to the SMTP functionality by using a beautiful dependency injection technique:

class Monad m => MonadSMTP m where
    putCrLf :: ByteString -> m ()
    throwSMTPError :: SMTPError -> m a

The protocol functions (sendCommand, etc.) will be in the MonadSMTP class:

sendCommand :: MonadSMTP m => Command -> m (ReplyCode, ByteString)

And the caller will provide a MonadSMTP instance that provides all of the requested capabilities.

This has several benefits:

• Isolates the pure code from side effects, simplifying reasoning
• Lets us operate on both a Handle and a TLS [Context](we need to do both to support TLS)
• Keeps subroutines from having to pass the SMTPConnection down the call chain. A Reader monad would provide the same benefit.
• Provides sufficient flexibility to implement, say, an smtp-conduit package

[jhickner]

This sounds great! I'll be looking forward to your pull request.

[alevy]

I'm assuming this never happened?

[shoooe]

@alevy Apparently not.

[jhickner]

This fork has has seen a lot more development progress. Check it out, it may have what you're looking for: https://github.com/avieth/smtp-mail