add Paging functions

[kellyjandrews]

When a payload is returned, it currently includes next or prev if there are additional results.

The user currently must call those routes directly - but this should provide .next() or .prev() functions to return the next/previous results with less effort.

[devintyler]

var account = new Syncano({accountKey: "ACCOUNT-KEY"});
var currentPage; // store current page to be used in next/prev calls

account.instance('INSTANCE').class('CLASS').dataobject().list()
    .then(function(res){
        console.log(res);
        currentPage = res;
  })
  .catch(function(err) {
    console.log(err)
  });


function prev() {
    if (currentPage.prev != null) {
        currentPage.prev()
        .then(function(res){
            console.log(res);
            currentPage = res;
    })
    .catch(function(err) {
        console.log(err)
      });
    } else {
        console.log("There are no more objects");
    }
}

function next() {
    if (currentPage.next != null) {
        currentPage.next()
        .then(function(res){
            console.log(res);
            currentPage = res;
    })
    .catch(function(err) {
        console.log(err)
      });
    } else {
        console.log("There are no more objects!");
    }
}