This allows jQuery to be successfully loaded from a web worker
since it does not have a document or window object by default.
Previously it would log about missing methods and properties.
Closes#228
This commit makes all interactions between Task and Worker look
like method calls. The worker now has a global `callTaskMethod`
function that it can use to call methods on the Task object. And the
Task can use `callWorkerMethod` to call methods on a global `handler`
object in the worker. The worker's initial `handler` actually contains
the `start` method, which the Task initially calls to kick things off.
Then the global `handler` gets replaced with whatever `handlerPath`
is specified by the Task. The worker then calls `workerStarted` on its
parent Task object.
This commit also gets rid of the `onProgress` method with the reply
semantics, favoring a more explicit interaction. When `snippetsLoaded`
finishes adding the snippet data, we call `loadNextPackageSnippets`
explicitly rather than returning a reply message.
