Refactor RACBlockTrampoline to accept a tuple of args

This just makes more sense for the use case, especially since we're
treating RACTupleNil specially.
Added -[RACStream reduceEach:] to reduce tuples
++phrasing
Add -zipWith: as a required RACStream primitive
Implement -zipWith: for RACSignal
Implement -zipWith: for RACSequence
Add support for appending to tuples
Implement +zip:reduce: in RACStream, remove overrides
Give +zip:reduce: a name, avoid crashing when the block is nil
Add -[RACSignal combineLatestWith:]
Refactor +combineLatest: in terms of +combineLatestWith:
Fix +zip: and +combineLatest: not returning 1-tuples
Make the documentation of reduce: variants consistent
Tests for +combineLatestWith:
Use isEqual: instead of ==
Remove the braces rustling @joshaber's jimmies
Better documentation for +zip: implementation
Style fixes
Whoops, use the temporary variable I added
Add a test for appending NSNull to a RACTuple