Merge Multirate into DSP

This is not even close to working yet. I'm just opening this pull request to get the process started.  I will continue to integrate when I can find time.

There are two separate sets of functionality in is pull request:
* Streaming multirate filtering
* Window based filter design

I already know that the DSP.jl developers are trying to get away from enums, which is used in the filter design code. I have no strong opinion on that code, and would happy if someone else wants to run with it.

The actual streaming code is my priority to fully merge. There's some redundant code for the different filt[!] methods. In the future it might make sense to combine them all into one with different work subroutines, but for now I will concentrate on getting the code to work within DSP.jl.