Clarify existence of FrameworkID in SUBSCRIBE calls.
Clarify existence of FrameworkID in SUBSCRIBE calls.

I wanted to clarify how `FrameworkID` should or shouldn't be set in
`SUBSCRIBE` calls.

This closes #205

reneploetz/mesos,abudnik/mesos,Gilbert88/mesos,dforsyth/mesos,gsantovena/mesos,reneploetz/mesos,Gilbert88/mesos,chhsia0/mesos,kaysoky/mesos,reneploetz/mesos,chhsia0/mesos,reneploetz/mesos,abudnik/mesos,andschwa/mesos,Gilbert88/mesos,kaysoky/mesos,gsantovena/mesos,reneploetz/mesos,andschwa/mesos,kaysoky/mesos,gsantovena/mesos,gsantovena/mesos,kaysoky/mesos,dforsyth/mesos,dforsyth/mesos,andschwa/mesos,gsantovena/mesos,kaysoky/mesos,chhsia0/mesos,jpeach/mesos,abudnik/mesos,jpeach/mesos,andschwa/mesos,reneploetz/mesos,kaysoky/mesos,abudnik/mesos,jpeach/mesos,chhsia0/mesos,jpeach/mesos,jpeach/mesos,abudnik/mesos,Gilbert88/mesos,Gilbert88/mesos,chhsia0/mesos,Gilbert88/mesos,andschwa/mesos,chhsia0/mesos,dforsyth/mesos,gsantovena/mesos,reneploetz/mesos,abudnik/mesos,chhsia0/mesos,kaysoky/mesos,dforsyth/mesos,andschwa/mesos,Gilbert88/mesos,jpeach/mesos,andschwa/mesos,dforsyth/mesos,jpeach/mesos,dforsyth/mesos,abudnik/mesos,gsantovena/mesos