Ignore target directories
UI: reference JSTL artifact available on Maven Central

The referenced javax.servlet.jsp.jstl:jstl:1.2 combination does not
exist on repo.maven.org. I've changed it to match the one deployed in
the latest version of the Sling Launchpad.
Added basic usage instructions
Adapt to released version of the sling taglibs

- Use the http://sling.apache.org/taglibs/sling URI taglib, according to
  [1]
- Remove the usage of sling:getProperties, I was unable to find it
  anywhere

[1]: http://sling.apache.org/documentation/bundles/sling-scripting-jsp-taglib.html