* Create asset filenames mapping on the build output

I use danethurber/webpack-manifest-plugin on Webpack production
configuration to create a file named `asset-manifest.json` which contain a
mapping of all asset filenames to their corresponding output file.

`asset-manifest.json` will be located at the root of output folder.

This'll resolve #600

* Add an explanation for ManifestPlugin

* Make webpack-manifest-plugin's version exact