Finalize API for basic statistics functions

This branch restarts the process of adding missing functionality to our basic statistics functions for skipping `NA` values while calculating statistics. The code is quite repetitive and can be DRY'ed out in a future run.

For now what I'd like to do is agree on what functionality these functions should offer. For now, I've taken every function I'm replacing and added a `skipna` keyword that allows one to skip over `NA` values. For `skewness` and `kurtosis`, this keyword has to be passed to the function that computes centers when they are not pre-specified, so the center is now also a keyword called `m`. (FWIW, I'm not a big fan of specifying centers that aren't the mean, so we might take that out. I'd argue it also doesn't belong in Base: neither R nor SciPy offer that functionality. I'm not sure why we do.)

Things we're not doing that R does:

* For `mean`, `std` and `var`, R also offers the ability to trim out extreme data points.
* For `median`, R also offers the ability to use the `lo` or `hi` median, which is simply the lower or higher value in an array with an even number of elements, instead of their average.

Unlike R, Julia Base expects that we will offer:

* Slices to compute column-wise, row-wise and other dimension-wise functions.
* Use of non-standard centers