Added methods to define default sort and limiter fields.

My first ever pull request!

OK, so with this pull, we can define default sorts and field limiters with defaultSortInfo() and defaultFieldInfo().  If the client passes a query string in the URL, they are completely overridden.

One thing I tried to do was to add the processing methods to check for invalid stuff, and allow you to supply the defaults in the same way as you specify them in the URL; e.g. instead of DESC/ASC you can simply supply 'fieldname' or '-fieldname'.

Field filters (as opposed to field limiters) seem to be more complicated, so I've leaving them for now.

So... how does it look?


I like this idea of allowing an optional default sort.

2 minor things:
  - [ ] Make sure that the docs say that this is optional.
  - [ ] Make sure to use a different example, since the default sort is based on entity id (even though you are using the reverse order).