Added functionality for using windows in periodogram functions.

Hi, 

Here is a initial pull request that adds some functionality for using windows in the periodogram functions. It is an initial PR to start a discussion on this topic and on the implementation. No tests and additional documentation have been added yet.

The way we should provide the window functionality is based on passing a function to welch_pgram and bartlett_pgram that accepts a single integer argument and return the window values in an array of that length. This array will be multiplied by the data when the call to periodogram is done.

I am not yet satisfied with the approach of passing an array of ones in the case no function argument is provided, as it was done for the spectrogram function and which I copied from.

Lets us discuss here how to best implement this functionality and I will modify the code as need for the final merge.

Regards, 

Davide

All of the built-in window functions return 64-bit floats, so I don't see much harm at the moment. If we were really concerned about Float32 there should be Float32 versions of those as well. I guess to play better with functions that might not return Float64 it might be best to use:

```julia
w = convert(Vector{Float64}, window(length(sig_split[1])))::Vector{Float64}
```

This can be revisited once JuliaLang/julia#210 is fixed, or if we decide to make windows into types.