Don't publish inactive items

Fixes #185

Changes:

*   `Tree`/`Document.find_item()` now raises an exception for inactive
items
* `Item.child_items` will now contain an `UnknownItem` if the child
item is inactive
Lowered minimum coverage to 99

The library is getting to the point where requiring 100% coverage is
making development harder.
Merge pull request #186 from jacebrowning/feature/inactive-publishing

Don't publish inactive items