WIP: Operations for DataArray that skip NA values

I've implemented a few operations on `DataArrays` that can skip `NA` values.  This behavior can eventually be tied to a flag to turn it on or off, but for now I just wanted to get something working.  I've implemented the approach outlined by @simonster in Issue #325 using the `NumericalExtenstions` package.

Also, since `NumericExtensions` doesn't work with `BitArrays` I've had to convert the `na` member to an `Array{Bool,N}`.  I realize this isn't very efficient right now, but it's hopefully a short-term solution.