Support extra_headers argument to Message to include custom email headers

Anything found in extra_headers is passed as-is to the underlying lamson message.
It will also override anything previously set by flask-mail.
Merge pull request #22 from rslinckx/patch-1

Add support for custom headers
Update change log and add new parameter to doc string
Merge pull request #20 from elasticsales/master

Fix HTML emails with attachments.
Updating github url in docs
Merge pull request #26 from philfreo/patch-1

Updating github url in docs
Merge pull request #24 from elasticsales/master

Allow sending mails with no body.
Update CHANGES
Bump version number to 0.7.4
Don't set Bcc header as some mail servers forward it to the recipient.