* Dropping Sink in favor of Write for writing mail

* Outstanding docs update
* Outstanding tests update
* Outstanding cleanup and refactoring

* Outstanding docs update
* Outstanding tests update
* Outstanding cleanup and refactoring

This is based on top of !6 to make use of current rust stable.

Peg 0.5 would create source code that triggers warnings
with current rust stable.

Upgrade to peg 0.6.

This is a first take.
There's a few open questions from my point of view:

* The return types in src/grammer/parser are getting long
  Would it help to define a type?

* The grammar used to be in a separate .rustpeg file
  The grammar syntax does not work well with autoindent.
  Peg itself still does so and has a bootstrap script.
  Is the cleaner separation worth the added build complexity?

Added mail IDs
MailService can refuse individual RCPTs
Mail queued is confirmed with an id

MailService now accepts Envelope, which is produced by Session
Made many internal Session methods private
Updated README.md with crate links

Can dump incomming mail to stdout
Worked out all the futures and streaming
It actually might work!