"limit" param for collections / ability to override top level json element name

You've created an amazing tool here, thank you for that. This pull request adds:

* The ability to pass a "limit" param to collections and have the return results be limited to N
* The ability to define a top level json element name in a model class (default is "resources")

I've got a project that has worked perfectly with "out of the box" sandman with the exception of these two features. Limit seems like a no-brainer to me. The top level json element feature was to give sandman the flexibility to be consistent with the payload formats that other APIs use.

If I'm missing any tests or anything else you'd like to see, let me know, I'd be happy to follow up on your feedback!

Thanks again for this cool tool,
Scott