#14 Add support for epic stories

[Anonymous]

Allow multiple stories to be categorised into epic stories, which is a finner grain of categorisation than themes.

Some properties of epic stories:
- Can be implemented across multiple sprints
- Typically contain multiple user stories
- Have a lifecycle (in progress, complete), but support for the lifecycle is not a critical feature

Often I need to view the set of stories that make up an epic story. Being able to sort the backlog or sprint via epic story name would give me this.