Add support for paging (cursor) and modified_after

Added support for pagination (cursor) and modified after

Usage modified_after:
PushBullet.pushHistory(function (err, res) { console.log(res); },
1414118516)

Usage cursor:
PushBullet.pushHistory(function (err, res) { console.log(res); },
undefined, 'MIfm92m93F23mwm9eg')