Allow Serializer Customization

Hi there,

I have some custom serialization requirements; however, currently the `CouchbaseClientExtensions` does not allow me to customize the `Newtonsoft.Json` serializer.

This patch allows serializer customization by placeing a static field on`CouchbaseClientExtensions.JsonSerializerSettings` that enables users of the couchbase extension class to customize the serializer.

Also added a static constructor to `CouchbaseClientExtensions` which maintains defaults.

Thanks,
Brian