[Bad backend support] Implement median reduction

[arogozhnikov]

Originated from patch #31

[arogozhnikov]

Multiple problems:

• mxnet and gluon don't have median (srsly??)
• torch can compute a median only over a single axis (we can reduce median to a sequence of medians)
• tf has no median per , but there is tf.contrib.distributions.percentile, contrib is not something to rely on
• cupy and chainer don't have median too

There is a way through sorting, but it is too slow.