Add `itemmap` and `itemfilter` to `dicttoolz`.

These complete the set of `{key,val,item}{filter,map}` functions.

Better examples and motivating use cases are welcome.

I first wanted `itemmap` to calculate a chain of mappings (so A maps to B maps to C) using a function like this:
```python
def isomorph(item):
    k, v = item
    return func(k), k
```

I think `itemmap(reversed, d)` to invert a dictionary is a common use case (as long as the key-values are bijective).  I think `itemfilter` is less useful, but it completes the set and is potentially useful for more complicated filtering.
I think I've copied `reverse_dict` into a half dozen `util.py` files by now.