BUG: Use zpk in IIR filter design to improve accuracy

Temporary fix for #2443, using private functions until a decision is made about how to arrange the functions for backwards compatibility, etc.

Examples:
17-pole digital Butterworth bandpass
Before:
![bad butter 17](https://f.cloud.github.com/assets/58611/1579769/ba499b88-51a6-11e3-8a44-60aee1ba17f7.png)

After:
![digital butter17](https://f.cloud.github.com/assets/58611/1579770/c638a3b2-51a6-11e3-8691-1cc5b9adb23f.png)

21-pole analog Butterworth lowpass at Wn=0.01
Before:
![signal butter 21 01 pz](https://f.cloud.github.com/assets/58611/1579783/5de5909e-51a7-11e3-8104-9e4c4fe5dd5b.png)

After:
![mine butter 21 01 pz](https://f.cloud.github.com/assets/58611/1579785/695665e8-51a7-11e3-8989-a6af4722ac9f.png)

cheby2 bandpass
Before:
![sloppy cheby2 freq resp](https://f.cloud.github.com/assets/58611/1579786/702f98f8-51a7-11e3-8cf1-5d25ee0a1a62.png)

After:
![better cheby2 freq resp](https://f.cloud.github.com/assets/58611/1579787/77a53908-51a7-11e3-8514-bbd0473358ad.png)