HystrixThreadPool shutdown methods

https://github.com/Netflix/Hystrix/issues/102
Hystrix.reset for lifecycle management

I'm changing the design from the previous commits so it's more abstract and can handle any type of resources needing cleanup, not just threadpools.

https://github.com/Netflix/RxJava/issues/45
make reset methods package private not public
Merge pull request #106 from benjchristensen/issue-102

HystrixThreadPool shutdown methods