Limit scope of CurrentThreadExecutingCommand

This is a tweak to: https://github.com/Netflix/Hystrix/issues/116

It should only start/stop on immediate sides of executeCommand() being invoked.
Merge pull request #137 from benjchristensen/CurrentThreadExecutingCommand-scope

Limit scope of CurrentThreadExecutingCommand
1.2.16
1.2.16
Gradle Release Plugin - pre tag commit:  'hystrix-1.2.16'.
Gradle Release Plugin - new version commit:  'hystrix-1.2.17-SNAPSHOT'.