add podspec for DspFilters

A Collection of Useful C++ Classes for Digital Signal Processing.

Here's a list of features:

Exclusive focus on IIR filters instead of boring FIR filters
Complete implementation of all "RBJ Biquad" Cookbook filter formulas
Butterworth, Chebyshev, Elliptic, Bessel, Legendre designs
Low Pass, High Pass, Band Pass, Band Stop transformations
Low, High, and Band Shelf filter implementations for most types
Smooth interpolation of filter settings, pole/zeros, and biquad coefficients to achieve seamless parameter changes
Representation of digital filters using poles and zeros
Realization using Direct Form I, Direct Form II, or user provided class
Fully factored to minimize template instantiations
"Design" layer provides runtime introspection into a filter
Utility template functions for manipulating buffers of sample data
No calls to malloc or new, great for embedded systems
No external dependencies, just the standard C++ library!