Revise contextPath to be '' OR '/context' but never '/'

Previously contextPath was set to '/' when it was serving on the root.
This created an issue when composing URLs in templates, for example:

    ${contextPath}/users

Would resolve to //users if serving on the root.
Improve Request & Response methods and allow context-sensitive redirects
Add CRUD context demo
Merge pull request #48 from decebals/context2

Fix contextPath definition problem and improve Request & Response context methods