Removed RACActiveSignals checking in RACDynamicSignal.
Fixed some tests.
Removed some tests that were testing implementation details.
Isolated multicasted subscription disposal test because ARC.
This isn't needed anymore.
Optimize -take: in case the count is 0

Comes from d87adfb (3.0-development).
Reconsider conditional expression on -take:
Merge remote-tracking branch 'origin/master' into remove-racactivesignals
Guard RACBacktrace with RAC_DEBUG_BACKTRACE instead of DEBUG.

Fixes #1463.
Merge pull request #1465 from ReactiveCocoa/dont-use-debug

Don't use DEBUG for RACBacktrace
Merge pull request #1336 from ReactiveCocoa/optimize-take

Optimize -take: in case the count is 0
Merge pull request #1135 from ReactiveCocoa/remove-racactivesignals

Remove RACActiveSignals
Skip adding disposables that are already disposed
Merge pull request #1478 from talko/disposablecherrypick

Cherry pick 3.0 change to skip adding disposables that are already disposed