Define 'triplestore' class for the Jena model object
Addresses issue #12.  This commit merely defines the class to inherit jobjRef class. Coercion is introduced into
all functions taking store/model as an argument, which converts the model back to a pure jobjRef class for
compatibility with .jcall interface, and a new('triplestore', model) call to all functions returing this object,
insuring consistent use of the class.  We can now add traditional S4 methods for the 'triplestore' class such
as show, summary, etc.