No longer writes BCC headers

This release is a bugfix release that changes existing behaviour. Hopefully this is a breaking change for only a few users!

For more information, see #14

Add inline attachments

Adds support for inline attachments thanks to @agustintorres.

v2.0.0 - General tidying, more error checking, parallel tests

General tidying, more error checking, and slightly better documentation.

Removes interfaces that should never have been part of this package (AttachmentEmailer and Emailer - clients needing them should define them). Because of this I've tagged this release as a new major version. I don't expect this will cause much, if any breakage to end users as they've probably never been used.

Uses t.Parallel() and t.Run() in tests, so Go 1.7 is now a minimum requirement.