Add Support for Partial Responses

[aidan-casey]

It would be nice to have the ability to specify which fields are returned by the Autotask API since this is supported functionality and would reduce response time for some requests. The problem we face here is representing partial entities to the user. This is a similar issue to that faced in #5.

[aidan-casey]

Perhaps one way to come at this would be to have partial entity classes (e.g. PartialContactEntity) that would be returned by requests that specify certain fields and a helper method on the default entity that returns fields that have changed since instantiation (e.g. ContactEntity::changed()).