replace R.mathMod and R.modulo with R.mod and R.rem

I set out to rename a couple of functions and ended up spending a long time on Wikipedia, GitHub, and elsewhere wrapping my head around these surprisingly tricky functions/operators.

#### mod

__Clojure:__

```
user=> (mod 42 5)
2
user=> (mod -42 5)
3
user=> (mod 42 -5)
-3
user=> (mod -42 -5)
-2
```

__Haskell:__

```
Prelude> 42 `mod` 5
2
Prelude> (-42) `mod` 5
3
Prelude> 42 `mod` (-5)
-3
Prelude> (-42) `mod` (-5)
-2
```

__Ramda:__

```
> R.mod(42, 5)
2
> R.mod(-42, 5)
3
> R.mod(42, -5)
-3
> R.mod(-42, -5)
-2
```

#### rem

__Clojure:__

```
user=> (rem 42 5)
2
user=> (rem -42 5)
-2
user=> (rem 42 -5)
2
user=> (rem -42 -5)
-2
```

__Haskell:__

```
Prelude> 42 `rem` 5
2
Prelude> (-42) `rem` 5
-2
Prelude> 42 `rem` (-5)
2
Prelude> (-42) `rem` (-5)
-2
```

__Ramda:__

```
> R.rem(42, 5)
2
> R.rem(-42, 5)
-2
> R.rem(42, -5)
2
> R.rem(-42, -5)
-2
```

@CrossEye can talk with greater authority on this, but i think the basic problem is that when the modulus is negative the output is ambiguous. that you have favored one possible outcome in the implementation doesn't lessen that ambiguity in the eye of the user. Also, we wanted a `mod` function constrained to *integers*. restricting the domains in these ways makes `mathMod` easy to reason about.