Create proper demo application

The demo app is just a basic create-react-app app with some customizable examples
showing off how react-datetime can be used.

Deploy the demo with `npm run deploy`

Access the demo (after deploying) from https://YouCanBookMe.github.io/react-datetime

Author: MrLeebo

demo/.gitignore

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+# See https://help.github.com/ignore-files/ for more about ignoring files.
+
+# dependencies
+/node_modules
+
+# testing
+/coverage
+
+# production
+/build
+
+# misc
+.DS_Store
+.env.local
+.env.development.local
+.env.test.local
+.env.production.local
+
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*

demo/README.md

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+# react-datetime demo app
+
+[Demo](https://YouCanBookMe.github.io/react-datetime)
+
+#### How to Start
+
+```bash
+npm start
+```
+
+#### How to Deploy
+
+Run "deploy" from the demo directory:
+
+```bash
+  cd ~/react-datetime/demo
+  npm run deploy
+```
+
+#### How to Run the demo with your local changes
+
+If you are working on some change and you want to use the demo to test them out, you have to link your local "react-datetime" directory to the demo:
+
+```bash
+  cd ~/react-datetime
+  npm link
+
+  cd demo
+  npm link react-datetime
+
+  npm start
+```