Changed MimeMessage.Sender to a MailboxAddress and added Resent-* header accessors

Fixes issue #17
Simplified message header changed/removed logic
Added convenience methods on MimeMessage to Sign, Encrypt and SignAndEncrypt
Bumped version to 0.17.0.0
Added rfc4262 as an implemented specification
Fixed compile bug on Windows
slight improvements to API docs
Worked on improving documentation a bit
Improved documentation a little more
Implemented some features that will help simplify writing an SMTP client
Fixed README
Fixed a variable initialization bug
Improved documentation for the various streams.
Regenerated xml docs
Fixed some typos in the docs
Don't change line endings for binary content

Fixes issue #18
Modified the various WriteTo() methods to all be cancellable.
Fixed Multipart.WriteTo() and modified FilteredStream to not keep track of position
Modified the MimeParser, MimeMessage.Load and MimeEntity.Load to be cancellable
Fixed documentation xml for the streams