Chebyshev (type I and II) filter prototypes

Also fix a bug in ZPK->SOS conversion with unequal numbers of zeros and
poles
Add elliptic filter prototype

Fixes #17
Tests for transformation of elliptic prototypes
Merge pull request #62 from JuliaDSP/sjk/cheby

Chebyshev (type I and II) and elliptic filter prototypes