Move attachments management from BaseMail.send to BaseMail.create_email_msg
Use or expression in get_attachments
Add *lang* optional parameter to _render_part and create_email_msg to
override the default get_language in BaseMail.
Rename *_get_mail_object* to *_get_mail_class*.
Add a real *_get_mail_object* in BaseMail which takes a template_name
and an optional context in parameters and return a MailClass instance.
Merge pull request #12 from thoas/refactoring

Excellent work @thoas, thanks!