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
Create .travis.yml
Update README.md
fix mark_seen method
Merge pull request #31 from SYNchroACK/master

mark_seen Method Fix