Add delay key stroke for filter

Added ability to add a delay for filter.

Test passed.

Usage:
<pre>
$scope.tableParams = new ngTableParams({
    page: 1,            // show first page
    count: 10,          // count per page
    sorting: {
        _id: 'desc'     // initial sorting
    }
}, {
    total: 10,           // length of data
    filterDelay: 750,    // default in milliseconds
    getData: function($defer, params) {
        Transaction.get(params.url(), function(data) {
            params.total(data.total);
            $defer.resolve(data.data);
        });
    }
});
</pre>