jsonalchemy: enhance extension parser behavior

* One extension could be either a importable string pointing to a class
  object or a function call that returns a class object.

* The extensions are now listed as quoted strings, either single quotes
  or double quotes due to some limitations of pyparsing.

Signed-off-by: Esteban J. G. Gabancho <egabancho@gmail.com>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>