turned HTMLBody into a bytes.Buffer to use it as a Writer for html/template
store all addresses as actual mail.Address instead of plain strings

moved parsing to when addresses are set on the message rather than during
message content creation.
variable name for golint
dropped implicit else statements
Revert "turned HTMLBody into a bytes.Buffer to use it as a Writer for html/template"

This reverts commit 4a15e52f78fea998683006a724e4eee848a6b501.
Merge branch 'mail-addresses' of https://github.com/dagoof/gophermail into dagoof-mail-addresses
Merge branch 'dev'