Implement attachment in Model, Serializer, Builder
Implement attachment in GUI
Change to HTML link for demo
Refactoring.
Should fix #306.
Fixes #311.
FIxes session logic
API for attachment
Merge pull request #313 from lowjoel/fix-311

Fixes #311.
Merge pull request #312 from angathorion/refactor

Refactoring and small fix
Merge pull request #314 from angathorion/session-logic-fix

Session now only keeps window size
Merge pull request #317 from evansb/attachment2

Attachment
Implement adding and removing both dependencies and subtasks.