Bulk api

Created clients for contact and CDO bulk API.  Docs and test coverage ready.
Perhaps it'd be preferable to include the identifier in the constructor for the custom object bulk client (and get rid of the `Bulk::identify()` method.  For instance, it'd be odd to have a bulk custom object client and, in the middle of bulk processing one type of custom object, use the same client to work with another type of custom object.