XDI Querier update

["Markus Sabadello" <markus.sabadello@xdi.org>]

According to today's discussion I updated the XDI Querier with the following changes:
- $$1, $$2 instead of $any$1, $any$2
- $$ instead of $all
- Constraints on literals are now expressed using cross-references as subjects

Example 7 now looks like this:

=giovanni
    +friend
        $$1
($$1/+age)
    $equals$or$lesser
        "26"
$$1
    +sister
        $$2
$$2
    +nationality
        $$
($$2/+nationality)
    $equals
        "Italian"

This example also demonstrates that the statements to which constraints apply may or not be selected for the result document:
- There's a constraint on the age of =giovanni's friends (must be <= 26), but those ages are NOT selected. They could easily be selected by adding a $$1/+age/$$ statement to the query.
- Then there's a constraint on the nationality of the friends' sisters (must be "Italian"), and those nationalities ARE selected, because the $$2/+nationality/$$ statement exists in the query.

Markus