Bug fixes if Work has no workExamples

This release provides from defensive checking in the event that the Work doesn't recognize any workExample properties.

TODO: investigate what sorts of situations cause this.

Multiple ISBNs for a Manifestation

Previously, Manifestation->getIsbns() would only return the first ISBN reference it ran across (an array with both ISBN13 and ISBN10), which could be confusing, especially if you found the Manifestation via ISBN and it wasn't what was returned by this method.

Now it should return all of the ISBN values found in the graph, assuming that the values are in an array property.

Bug fixes

This release deals with some bugs around scenarios where a graph typed literal is in a language that is outside of the preferred languages would return null if you tried to get the property value.

It also prevents caching of a Work that 404s if accessed through Manifestation->getExampleOfWork() which result in a non-functioning Work with no way to really know that it was broken or fix it.

Add a license

Forgot to add a license prior to releasing v0.1.

Initial release

v0.1

Update README.md