added functionality to create HITs using HITLayouts

To provide a less technical user interface to generating HITs you can use the requester UI when collaborating on mturk project with non-coders. A properly formed HITLayout yields a LayoutID when clicking on the project name links at https://requestersandbox.mturk.com/create/projects/. These layouts can additionally be supplied parameters in the form of ${parameter}, and the added functions handle this. 
Functions the same as HIT.create(), but takes an additional third argument as a JSON object containing parameters and values as {name:value} pairs.