simplify controller structure

restructure broadcast stream controller to resemble that of the activity
stream controller by using a promise made in AuthService
use serverTimeOffset

note: the offset turns out to be non-negligible (e.g. ~17 seconds on my
computer)
remove unnecessary promise.then
switch from filteredBroadcasts object to array
eliminate Date.now()

Date.now() is not supported < IE9
fix merge conflicts
Merge pull request #10 from reebalazs/broadcast_stream

Broadcast stream