Rename server/index.js to server.js.

Then it can be started with `npm start`

# Please enter the commit message for your changes. Lines starting
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# rebase in progress; onto 40292c5
# You are currently editing a commit while rebasing branch 'feature/refactor-gulp' on '40292c5'.
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# Changes to be committed:
#	modified:   package.json
#	renamed:    server/index.js -> server/server.js
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Document dir structure in README.
Add commands to README.
Update scripts in package.json.