Fixes issue #93 #97 #98 #99

1) Implementing clean shutdown of client. Moved away from Timer to ScheduledExecutorService as it gives better control of shutdown by interrupting the running thread, which is what we want.
 2)
Using ephemeral ports for eureka server mock. Thanks @elandau !
Incorporating review comment.
Added testcase for app group.
Merge pull request #100 from NiteshKant/master

Fixes issue #93 #97 #98 #99