[WIP} Added a WIP array.mean() implementation.

Related to issue #828 and some others.
I honestly learned this idiom looking at [this repository](https://github.com/numba/numba/blob/master/numba/typing/npydecl.py#L194), so i thought it would fit-in well.  It also make sense for min()/max() and a similar for argmin() and argmax() which I was considering during this implementation.