Bugfix: client_select was returning a list

And we were making poor use of it!
Basic JSON output for dataTables
New features in the web interface

* by putting a .json extension to an url (/clients/list.json) you get
the data in JSON format
* this is used by dataTables, that have been integrated as well
* at the moment a lot of things are hardcoded and not modular (after
some UI styling, this will be the next step)
* updated README.md to reflect latest changes