Add `slice` method to enumerator.
Add better constructor syntax. Fixes #11
Fixes for `slice` to support negative ranges.
BC Break - Treat each/collect differently.

Each gives the value to change, collect uses the return value as the new value.
Add `fill` method for enumerator.
Add Countable to PrettyArray. Fixes #12

BC Break! You can no longer call `count` statically on PrettyArray.
~~~ php
PrettyArray::count($foo); // breaks!
~~~