Mnist torch example

[suiguoxin]

• add torch mnist example
• change links for mnist examples in README

[suiguoxin]

Difference from mnist-tensorflow example:

• in mnist-tensorflow example, it reports the intermediate result every 100 batches, no matter the size of batch
• in this example, it reports the intermediate result every time it traverses the training data

[QuanluZhang]

could you also add it into IT?