added support for automatic pagination traversal

In response to [Issue #37](https://github.com/ejholmes/restforce/issues/37), this code introduces automatic pagination traversal. To do this, I changed `Restforce::Collection` from subclassing `Array` to mixing in `Enumerable`. This change made it easier to lazily request the next page, in lieu of making all of the requests at once. Please let me know if there are any issues, I would be happy to address them.