Add reverse binary exercise

Based on [Spotify Puzzles exercise](http://labs.spotify.com/puzzles/)
The type hint here does nothing. Type hints are only useful when there's interop in the lexical scope -- all you're doing is passing `x` to a normal function, so there's no interop here. The hint on your `str-reverse` function is also unnecessary for the same reason.

The hints on `decimal->binary` and `binary->decimal` on the other hand do use interop with `x`; in these particular cases you still don't need the type hint because there's only one method with the given name on the `Integer` class, but that's not obvious. `lein check` can tell you if your code has reflection (and therefore lacks needed type hints).