Add x_mailer & boundary, fix sender field in Email download

The "x_mailer" and "boundary" fields were missing from Email download.
The "sender" field was incorrectly being populated with the contents of "reply_to".
Add x_mailer & boundary to STIX Email import
Merge pull request #2 from brlogan/patch-11

Add x_mailer & boundary, fix sender in STIX Email import/export