[Incomplete] Filtering refactor, add IIR

I was thinking about this as well, just wasn't sure how many filters we'll eventually end up with.
But I would still prefer to keep a ```filter``` subpackage. 
There might be use cases where you for example only want to use a small portion of the breeze.signal lib and having logical subpackages will prevent the namespace from being polluted with unused imports / objects.
consider you only want to use ```fourierFreq```or ```convolve```, so you would ```import breeze.signal``` and you'll also automatically have all the filters etc. in your namespace

my suggestion: we should only include "core" functionality (like fourier, convolve ...) in the breez.signal root package and group everything else into logical subpackages (filter, wavelet, bss ...)
with package objects we can still use a single "import breeze.signla.filter" statement to bring all the filters in scope.

just my thoughts though, I'm open for everything else