Fixing Action.__repr__()
More descriptive Action names.

- Renamed the classes (and their tables) that inherit from the Action model:
  - View => ViewPost
  - Expand => ExpandPost
  - Collapse => CollapsePost
Fixing typo in CollapsePost.verb
Adding Action models to assembl.models
Adding method TableOfContents.serializable()
Merge branch 'develop' of https://github.com/ImaginationForPeople/assembl into develop
Adding serialization methods to models.