Fixed incorrect normalized cutoff frequency computation in butterworth.

The nomarlized cutoff frequency for Butterworth filters should be:

Wn = cutoff / (0.5 * samplerate)

But I was using:

Wn = cutoff / 2.0 / samplerate
Added btype argument to butter.
Merge pull request #21 from moorepants/issue-20

Fixed incorrect normalized cutoff frequency computation in butterworth.