Added test for task added/update event.
Added test for task added/update event.

This change adds a basic test to ensure that a client is able
to subscribe and receive task added/updated events.

Review: https://reviews.apache.org/r/48880/

zmalik/mesos,abudnik/mesos,reneploetz/mesos,jpeach/mesos,reneploetz/mesos,abudnik/mesos,andschwa/mesos,yuquanshan/customizedMesos,jpeach/mesos,craimbert/mesos,andschwa/mesos,Gilbert88/mesos,craimbert/mesos,jpeach/mesos,craimbert/mesos,andschwa/mesos,abudnik/mesos,craimbert/mesos,yuquanshan/customizedMesos,Gilbert88/mesos,Aman-Jain-14/customizedMesos,jpeach/mesos,zmalik/mesos,chhsia0/mesos,neilconway/mesos,kaysoky/mesos,asamerh4/mesos,kaysoky/mesos,gsantovena/mesos,gsantovena/mesos,neilconway/mesos,chhsia0/mesos,verizonlabs/mesos,Aman-Jain-14/customizedMesos,Aman-Jain-14/customizedMesos,yuquanshan/customizedMesos,reneploetz/mesos,abudnik/mesos,asamerh4/mesos,neilconway/mesos,dforsyth/mesos,dforsyth/mesos,abudnik/mesos,kaysoky/mesos,chhsia0/mesos,jpeach/mesos,Aman-Jain-14/customizedMesos,zmalik/mesos,Gilbert88/mesos,kaysoky/mesos,Aman-Jain-14/customizedMesos,gsantovena/mesos,reneploetz/mesos,andschwa/mesos,yuquanshan/customizedMesos,Aman-Jain-14/customizedMesos,zmalik/mesos,verizonlabs/mesos,Gilbert88/mesos,Gilbert88/mesos,Gilbert88/mesos,kaysoky/mesos,dforsyth/mesos,Gilbert88/mesos,Aman-Jain-14/customizedMesos,dforsyth/mesos,chhsia0/mesos,craimbert/mesos,reneploetz/mesos,andschwa/mesos,yuquanshan/customizedMesos,zmalik/mesos,abudnik/mesos,verizonlabs/mesos,kaysoky/mesos,dforsyth/mesos,reneploetz/mesos,asamerh4/mesos,zmalik/mesos,yuquanshan/customizedMesos,zmalik/mesos,gsantovena/mesos,dforsyth/mesos,gsantovena/mesos,neilconway/mesos,abudnik/mesos,asamerh4/mesos,chhsia0/mesos,asamerh4/mesos,gsantovena/mesos,verizonlabs/mesos,craimbert/mesos,neilconway/mesos,chhsia0/mesos,reneploetz/mesos,andschwa/mesos,dforsyth/mesos,verizonlabs/mesos,verizonlabs/mesos,asamerh4/mesos,neilconway/mesos,kaysoky/mesos,verizonlabs/mesos,yuquanshan/customizedMesos,neilconway/mesos,asamerh4/mesos,craimbert/mesos,andschwa/mesos,gsantovena/mesos,chhsia0/mesos,jpeach/mesos,jpeach/mesos