make isDependent(src,dest) overridable from user code

This makes the internal isDependent() overridable via ```Intercooler.isDependentFunction = func(src, dest) { return bool; }```.

The point of this is that some users might want more fine-grained control over how URLs are matched against each other.

By default, Intercooler considers ```/model/123``` to be a dep of ```/model/1```. This is a bit awkward, and might trigger multiple refreshes.

I'm using the attached patch to do this:

    Intercooler.isDependentFunction (src, dest) ->
      unless src && dest
        return false

      sa = src.split("/")
      sd = dest.split("/")

      sd.slice(0, sa.length).join("/") == sa.join("/")

which basically checks that the individual path elements are equal, and not just the start of the string.

Not sure if this is something you'd want to consider.