Merge pull request #348 into master

We want to be able to plot multi-frequency records, with each signal
at its native frequency.  To see the individual samples, try:

    import wfdb
    r = wfdb.rdrecord('sample-data/03700181', smooth_frames=False,
                      sampfrom=5000, sampto=5100)
    wfdb.plot_wfdb(r, time_units='seconds', sig_style=['|-'])

Compare to the smoothed version:

    r = wfdb.rdrecord('sample-data/03700181',
                      sampfrom=5000, sampto=5100)
    wfdb.plot_wfdb(r, time_units='seconds', sig_style=['|-'])

A more dramatic example:

    r = wfdb.rdrecord('sample-data/wave_4', smooth_frames=False,
                      sampto=10)
    wfdb.plot_wfdb(r)

contrasted with:

    r = wfdb.rdrecord('sample-data/wave_4', sampto=10)
    wfdb.plot_wfdb(r)

To see the signals synchronized with annotations stored at different
frequencies, try:

    r = wfdb.rdrecord('sample-data/03700181', smooth_frames=False,
                      sampfrom=5000, sampto=5625)
    for ann, f in (('gqrsh', 500), ('gqrsl', 125), ('sqrs', 250)):
        a = wfdb.rdann('sample-data/03700181', ann, shift_samps=True,
                       sampfrom=(5000*f//125), sampto=(5625*f//125))
        wfdb.plot_wfdb(r, a, time_units='seconds',
                       title='annotator %s' % ann)

Author: Benjamin Moody