Rename ResizingPolicy to AutoscalerPolicy

[aledsage]

It's been proposed on brooklyn-dev that Autoscaler is a much easier to understand name than ResizingPolicy (https://groups.google.com/d/msg/brooklyn-dev/eLG8-EahIpw/4YLU6jvLoo4J).

Let's rename it.

We currently have two such policies:

• ResizerPolicy looks at a metric on a resizable entity, and compares it against some thresholds
• ResizingPolicy is subscribes to "pool cold" and "pool hot" events, to trigger the resizing

We should look at merging these, as there is big overlap. Alternatively, come up with good names that indicate their differences better.

[aledsage]

Working on this (see https://github.com/brooklyncentral/brooklyn/pull/199).

Next up: look at merging ResizerPolicy and ResizingPolicy. And look at renaming notification-sensor names.