Recognize inline images as attachments

When parsing attachments allow content-disposition to take either of two
values: 'attachment',  or 'inline'. The latter appears to be used by
iPhone when sending images.
Merge pull request #28 from gauden/master

Recognize inline images as attachments
Add 'Reply-to' to headers.
This might be not good since it's a quick fix. Need to consult the main
developer about his.