Add a bucketing method with equal cumulative bucket duration

[pzelasko]

Hopefully resolves the issue #364

@danpovey @csukuangfj please check this out and LMK if it helps with the sawtoothed loss

To use it, add the argument:

sampler = BucketingSampler(
   ...,
   bucket_method='equal_duration',
)

[pzelasko]

I also double-checked that shuffling works as intended in the BucketingSampler.

[csukuangfj]

Thanks! Will try it and post the result here.