Set versionModel._id to null when creating a copy of the shadow document

Hi there,

I've updated the ```versionModel._id = null``` as discussed in https://github.com/saintedlama/mongoose-version/pull/7 and https://github.com/saintedlama/mongoose-version/issues/9

I've ran the provided test cases and performed tests to ensure a shadow document is created for each edit of the original document.

I've tested this in mongoose version 3.8.13

Can you please merge?