Added sections on variables and template variables to the JSON Model page

I realized when I created the https://wiki.oasis-open.org/xdi/JsonModel proposal, I forgot to include anything about variable syntax. Due to the JSON model only needing to use ( ), [ ], and < > brackets, it conveniently leaves { } for variables. See:

  https://wiki.oasis-open.org/xdi/JsonModel#VariableSyntax

I also included section on template variables, which are only for documentation, but still important to standardize to make developer's lives easy. The proposal is to use <--variable-->. See:

  https://wiki.oasis-open.org/xdi/JsonModel#TemplateVariableSyntax.28Informative.29

Again, please look this over, play with it, and send any questions about the JSON Model or syntax to the list -- or be prepared to bring them to Friday's telecon, since we really want to move from there into implementation (we have a lot of wiki pages and other docs, not to mention CODE, to begin updating).

Thanks,

=Drummond