Add getUserAddress to HackSessionContext
Remove getAddress function (superceeded by getPublicId for now)
Always make the grain's address be the `from` in the smtp envelope.

Also remove old logic for replacing `from` header in messages, and
instead allow either the grain's address or the user's sandstorm
registered address to be used.
Merge pull request #26 from jparyani/mail-fix-from

Allow sandstorm registered email addresses in outgoing mail