Added prototype for Document.reorder().
Merge branch 'master' into reorder
Added placeholder tests for import functionality.
Implemented `importer.new_document`.

Still need to implement `importer.add_item` and command-line interfaces.
Added a new exception message for missing parents.
Added more importer tests.
Tests are passing for importer.new_document().
Tests are passing for importer.add_item().

The CLI is remaining.
Cleanup.
Merge from 'master' branch.
Merge branch 'master' into reorder
Added settings.REORDER
Implemented ordering and added integration tests.

TODO:

- remove comment to properly classify integration tests
- add unit tests for coverage
Cache clearing decorators aren't shared.
Heading are now kept when reordering.
Added failing test.
Added missing assert.
Git attributes.
Split up doorstop.core.base into base and types.
Added a placeholder Level class.