Clean up patch object to only contain modifications

[mrichar1]

Inspired by #59 I've been thinking more about how we handle PATCH data.

When making a patch, it may be useful to be able to 'clean' the patch object, so that it only contains modifications.

This is useful for the workflow where a record is fetched from the API and stored, then the get getter returns a copy, which then has a single attribute modified, and is then used in a patch action. It makes sense for the patch to only contain the changed attribute, and not be the whole store object (this prevents accidental pushes of stale data to the store, and is more efficient).

At it's simplest, there should be a config option, which, if enabled compares attributes in the patch and the store, and keeps only those that are 'different'.

To begin with, this should only consider attributes, but longer term we may want to add further functionality:

1. look at changes to relationship links (e.g. modifying _jv/relationships/author from {type: 'author', id: '1'} to {type: 'author', id: '2'}
2. look at changes to relationship data (where 'followed' relationship data in the the root is modified, generate a 2nd patch targeted against the related object's URL).

The first part should be easy to implement, so will begin work on a PR.

[mrichar1]

I've kept this open as it's likely that '#63 probably isn't the 'right' way to do this. Instead a better option would be to have cleanPatch method available for use on a record prior to passing it in as the argument to patch. This would make is useable on a per-record basis, and also make it more flexible in what it can do.

This relies on #65 being addressed first of all, to make helper methods generally available.