Implement hacky support for e-mail sending and receiving.

This implementation is a stop-gap to get e-mail running without persistent capabilities
nor the Powerbox. The Sandstorm front-end acts as an SMTP server. Each grain is assigned
a random "public ID" which acts as its e-mail address. This means that each grain can
have no more than one address, which it uses both for sending and receiving.

This is a combination of 28 commits:
* Initial version email.capnp
* Add initial version of smtp server
* Re-export SandstormApi and SessionContext capability to child process of legacy-bridge
* Hack to get basic email context working. Ignore powerbox for now
* First semi-working version of mail server RPC
* Update email Cap'n Proto spec. It's almost finalized
* Add better mailparsing and improve email writing in legacy-bridge
* Add `email` field to Grains collection which decides what emails get
  sent to which grains. Only settable in `meteor mongo` atm.
* Fix typo in gitignore
* Add a few more headers to legacy bridge. Also add default Message-Id
* Add initial version of outgoing email using sendgrid
* Add settings file
* Rename EmailHackSession to HackSession
* Remove explicit check for initializing HackSession
* Change to using publicId for emails
* Switch to using built-in mail package
* Make sending/receiving email much more robust, resolving most of the TODOs
* Fix to prevent empty fields from being printed in email messages
* Make comparisons case insensitive for email addresses
* Fix bug in getting publicId
* Change to using a promiseFulfillerPair for passing SessionContext
* Random change in npm-shrinkwrap
* Add copying *.capnp to node_modules task back in
* Add getAddress method to EmailHackSession
* Change to use `/var/socket-api` instead of fd=4 in http-bridge
* Add multipart handling for emails
* Change the api socket to be in /tmp instead of /var
* Fix typos in generating publicId
Merge Jason's e-mail implementation.

Conflicts:
	src/sandstorm/sandstorm-http-bridge.c++
Tweak implementation of e-mail.

Conflicts:
	nginx-example.conf