Added table distinct and fixed links in joins

You are now able to get the distinct values of a table column:

$tripod->getDistinctTableColumnValues($tableSpecName, $fieldName);

=> array(
    'head'=>array(
        'count' => 3
     ),
     'results' =>array('Foo', 'Bar', 'Baz')
)

This release also fixes an issue where "value":"link" in a join definition in a tablespec would supply the URI of the top resource. It now supplies the URI of the current resource