fix some problems and add online demo

[dzylikecode]

• add Makefile
• add some files that were not included
• fix a derivation issue during backpropagation
• add an online demo

[KorigamiK]

I believe you changed too much here. I would suggest you to revert back some changes and cherry pick only those commits that are relevant to this project only.

Another thing, I think the online demo is down for now.

[dzylikecode]

Okay, thanks for your advice.

[dzylikecode]

Here is the online demo: https://dzylikecode.github.io/Neural-network/