RxJavaObservableExecutionHook

Allow a plugin to intercept invocations of an Observable via its subscribe method.

Use cases:
 - extra logging or metrics for start, return and error events when subscribing
 - decorate (or replace) an 'onSubscribe' function with additional behavior
 - decorate an Exception
 - decorate a Subscription

This is modeled after Hystrix Plugins: http://netflix.github.com/Hystrix/javadoc/index.html?com/netflix/hystrix/strategy/executionhook/HystrixCommandExecutionHook.html

Javadocs will be at http://netflix.github.com/RxJava/javadoc/
Merge pull request #224 from benjchristensen/execution-hooks

RxJavaObservableExecutionHook