Adding books requires identifiers

It seems that the library currently requires specifying either an ISBN or LCCN identifier in order to be able to create a new book, which differs from the behaviour of the web form for the book creation, [where the identifiers are optional](https://openlibrary.org/books/add).

To reproduce this should work just fine

```
from olclient.openlibrary import OpenLibrary
import olclient.common as common
ol = OpenLibrary()
book = common.Book(title="My test book title", authors=[common.Author(name="Testy McTestFace")], publisher=u"Publisher", publish_date=u"2018")
new_book = ol.create_book(book)
```

This will [trigger the value error](https://github.com/internetarchive/openlibrary-client/blob/master/olclient/openlibrary.py#L765).

I'm not sure if this is expected behaviour that was designed to work like this, or whether the `openlibrary-client` is just out-of-sync with how the web-form behaves? If it's the latter I'd be happy to give it a shot to adjust it according to the web form🙂

Start at the value-error path in olclient/openlibrary.py around line 765 and reproduce it with the Book example in the issue. Compare the client behavior with the linked web-form behavior and determine whether identifiers should be optional. Done means creating a book without an ISBN or LCCN works consistently with the intended behavior, with appropriate test coverage.

Written by the indexing model from the issue text.