Changes to support calculation wall-clock time taken by a set of hystrix commands

Added in support for calculation wall-clock time taken by a set of hystrix commands usually within the scope of a request.

Use-Case:
We develop and own a service say postUserComments which in turn depends on a set of independent services say
1. detectSpammyURLInContent
2. detectAbuse
3. postToTwitter
4. buildSecondaryIndices
5. ....

All but buildSecondaryIndices are external services that are beyond the scope of current service/system. These are executed in parallel or in sequence depending upon service semantics.

The added feature will help capture true "wall-clock" overhead of postUserComments service, which can be used by service implementations to monitor/alert and optimize on SLA's guaranteed by services.
The main issue with `System.currentTimeMillis()` is that it's not monotonically increasing.

 http://stackoverflow.com/a/351571/139010