HystrixAsyncCommand -> HystrixFutureCommand

As per https://github.com/Netflix/Hystrix/issues/321#issuecomment-57698763
FutureCommand.run() -> start()

Also getFallback() -> startFallback()

As per https://github.com/Netflix/Hystrix/issues/321#issuecomment-57700385
onFailureResumeWithFallback -> resumeWithFallback

As per https://github.com/Netflix/Hystrix/issues/321#issuecomment-57698845
HystrixExecutableInfo -> HystrixInvokableInfo

As per https://github.com/Netflix/Hystrix/issues/321#issuecomment-57702442
Remove HystrixFutureCommand

Decided that we don't want to commit to this yet since it has an awkward interface until RxJava offers a scalar Observable solution.
Merge pull request #338 from benjchristensen/api-design

API Changes as per Design Review