Expand import functionality to understand export formats.

TODO:

- [x] Wait for #45
@jacebrowning I just tried this and I get a prefix of 'REQ2' which causes the tests to error out.

Is REQ2-001 supposed to be a second document? (or is your ID.prefix a bit off?) 

If it is supposed to be a second document, how is document hierarchy maintained through export/import?

```
  File "C:\Users\theovoss\Documents\GitHub\doorstop\doorstop\core\importer.py", line 120, in import_csv
    add_doorstop_items(rows[1:], rows[0], True)
  File "C:\Users\theovoss\Documents\GitHub\doorstop\doorstop\core\importer.py", line 207, in add_doorstop_items
    new_document(prefix, tree.root + os.sep + prefix)
  File "C:\Users\theovoss\Documents\GitHub\doorstop\doorstop\core\importer.py", line 53, in new_document
    raise exc from None
  File "C:\Users\theovoss\Documents\GitHub\doorstop\doorstop\core\importer.py", line 50, in new_document
    document = tree.new_document(path, prefix, parent=parent)
  File "C:\Users\theovoss\Documents\GitHub\doorstop\doorstop\core\tree.py", line 20, in wrapped
    result = func(self, *args, **kwargs)
  File "C:\Users\theovoss\Documents\GitHub\doorstop\doorstop\core\tree.py", line 31, in wrapped
    result = func(self, *args, **kwargs)
  File "C:\Users\theovoss\Documents\GitHub\doorstop\doorstop\core\tree.py", line 216, in new_document
    self._place(document)
  File "C:\Users\theovoss\Documents\GitHub\doorstop\doorstop\core\tree.py", line 177, in _place
    raise DoorstopError(msg)
nose.proxy.DoorstopError: no parent specified for REQ2 (@\REQ2)
-------------------- >> begin captured stdout << ---------------------

ID Text is REQ001
Corresponding prefix is REQ

ID Text is REQ003
Corresponding prefix is REQ

ID Text is REQ004
Corresponding prefix is REQ

ID Text is REQ002
Corresponding prefix is REQ

ID Text is REQ2-001
Corresponding prefix is REQ2
```