Preparation for 4.0 RTM.

- Added new test cases for handling malformed event streams.
- Added new test case for SendGrid's new event test payload.
- Added Portable.Mail.MailAddress.ToString() to fix a failing test.
- Added new SmtpId property to the EventData object.
- Fixed a bug where headers were not being added to a message properly.
Update NuGet packages to support WP 8.1