Update Javadocs for plugin configuration.

This makes the Javadocs comply with changes done in https://github.com/Netflix/Hystrix/commit/bfb66969baaa38a16e9dfce872e6ad6a4012d443
Remove TODOs made invalid by HystrixPlugin changes

Plugins can not be modified after registration so caching is okay.
Change Throwable to Exception

Generally not good practice to be catching Throwable so changing to Exception.
Remove unused code.
jettyRun support for running webapps via gradle
HystrixCommand Execution Hooks

Execution Hooks via Plugin https://github.com/Netflix/Hystrix/issues/10
ExecutionHook: Change onSuccess to onComplete

I think onComplete is clearer since the response can be either from run() or getFallback()
Additional javadocs for ExecutionHook/EventNotifier
Merge pull request #71 from benjchristensen/execution-hooks

HystrixCommand Execution Hooks https://github.com/Netflix/Hystrix/issues/10