add timing variation as a parameter

[etosch]

Right now everything gets launched at once. However, we may want to release HITs in bursts. Consider the following scenarios.

• First add the ability to set a max time for the entire survey. This is an extension of the HIT maxtime policy.
• Next add a kind of concentration parameter -- we could have a series of options, including: FRONT, which front loads the HITs (posts all at once), UNIF, which posts the HITs uniformly over the time parameter, ADAPT, which posts HITs adaptively, according to when it believes the response rate will be maximized. Note that we need (dis)qualifications back for this to work.

For AMT, posting/availability is conflated. More recent HITs are listed first. Forums outside AMT post details on recent HITs. On another system, there might be a different relationship between the availability of a task and whether/how people are notified about it. This should be abstracted out in Runner.

[etosch]

not doing maxtime -- just the concentration parameter (which should maybe be automatically set later on)