More ways to merge HotSignals; Update to carthage 0.4

Addresses #1647.

In terms of new functionality, this PR adds a `merge` overload to `ColdSignal` which merges a `ColdSignal` of `HotSignal`s, as well as the `merge(SequenceType)` class method and `mergeWith(HotSignal)` instance method to `HotSignal`, which are essentially conveniences for the former. And I added some tests to check that these work.

I've also updated the project to use the new Carthage 0.4 directory structure (mostly because I have 0.4 installed and I needed to build the project ¯\\\_(ツ)\_/¯ ). But this is all confined to 1 commit so I can easily revert it if we don't want to update yet.

I have a few small things that I'm unsure about here: 
- Whether the `merge(SequenceType)` class method should be considered a "Convenience constructor" or a "Method for combining multiple signals". I put it in the latter extension, but I could see the case being made for either.
- If `ColdSignal`'s `mergeMap() -> ColdSignal` would need to be changed now that there is an overload that returns a `HotSignal`. I suspect it will be, but I'm not sure what the best solution is.
- I made my best guess for the generics mumbo-jumbo in HotSignal.swift:573 but it doesn't feel exactly right.

And of course please do let me know if anything else doesn't look right!
In general, I think we should [shy away from convenience operators](https://github.com/ReactiveCocoa/ReactiveCocoa/issues/1620). I think it makes sense compared against `combineLatestWith`, because that method is actually the primitive upon which the multiple-signal version is built.