Implement sorting

[SphtKr]

Per the spec, sorting of requests may be implemented via the query string. It appears we could pick either of the two formats, the latter is required for sorting on related objects' properties...which we probably wouldn't support initially anyway, but might be simpler to implement.

This will probably be implemented via an ActionFilter. Probably a separate one, but would there be any benefit to combining it with the one added in #16?

[csantero]

I am leaning towards putting this in a separate action filter too. I can't think of any good reasons why a user wouldn't include both filters, but it's still a cleaner design.

[SphtKr]

It occurred to me that one might want to omit sorting or querying for performance reasons...lack of indexes etc. could make sorting or filtering perform very poorly.

In fact, it now occurs to me that it might be good to find a way to let the user blacklist or whitelist fields for sorting (or filtering, for that matter). Some of my model objects have computed properties and sorting on one of those (if it even worked) would kill my app.