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 from 'master' branch.
Removed .orig file.
Merge pull request #43 from jacebrowning/importer

Add import functionality.