first pass at -mergeConcurrent:
break out of recursive locks
actually remove the subscribable from the active subscribables
fix off-by-one with active subscribables
dequeue from the front of the array instead of the back
actually add the subscribable to the active subscribables
note that 0 means unlimited
tests for -mergeConcurrent:
smaller lock scopes
completion tests
Merge branch 'master' into merge-concurrent

Conflicts:
	ReactiveCocoaFramework/ReactiveCocoaTests/RACSubscribableSpec.m
style
implement all the merge variations in terms of -mergeConcurrently:
tests for merge
more documentation
docs
work on taking an arbitrary block
++xcconfigs for armv7s
start with the current value of the text fields
mergeConcurrent => merge