Add Mnist Data.

[rilmayer]

I tried to write code about Mnist(http://yann.lecun.com/exdb/mnist/). I think we will be able to use Mnist Dataset.

[rilmayer]

I am very thankful because I have not had the opportunity to be reviewed the code as usual. I would like to fix if there is something else!

[rilmayer]

Thanks. I fixed where you commented.

[rilmayer]

Sorry for some minor fixes, but I think I could have fixed all where you commented.

[rilmayer]

Thanks, I have fixed the code you commented. Please check my comments for some fix.

[kou]

Thanks! Now, we can use it in Red Chainer for MNIST dataset.

We can work on exception classes as another work.

[rilmayer]

Thank you for having reviewed, and for having merged. I have learned a lot, so I want to commit more.

And Thanks for some fix to my code.

[kou]

Great! We should support more datasets! For example, we can find some target datasets at https://github.com/red-data-tools/red-datasets/issues .