BUG: Fix the automatic frequency selection of scipy.signal.bode.
The automatic selection of the frequencies could miss zeros or resonance peaks, so use the existing function scipy.signal.findfreqs instead of _default_response_frequencies.

Specific changes:
* Modifed the function scipy.signal.bode to use scipy.signal.freqs.
* Added a docstring to scipy.signal.findfreqs.
Merge pull request #432 from WarrenWeckesser/bode-freqs

BUG: signal: Fix the automatic frequency selection of scipy.signal.bode