installation: explicit jQuery plugin versions

• Specifies versions for jquery.form.js, jquery.MultiFile.js, jquery.dataTables.min.js and other dependencies. (addresses #11) (PR #2655)

• Adds -N for wget statements to override downloaded files so that that would not pile up.

• Adds -u to unzip calls to avoid queries in case files exist.

• TODO json2.js does not provide any versioning tags on GitHub, so it still comes in the latest version.

Reviewed-by: Tibor Simko <tibor.simko@cern.ch>