New ?_shape=objects/object/lists param for JSON API (#192)

New _shape= parameter replacing old .jsono extension

Now instead of this:

	/database/table.jsono

We use the _shape parameter like this:

	/database/table.json?_shape=objects

Also introduced a new _shape called 'object' which looks like this:

	/database/table.json?_shape=object

Returning an object for the rows key:

	...
	"rows": {
		"pk1": {
			...
		},
		"pk2": {
			...
		}
	}

Refs #122

datasette/templates/row.html

@@ -22,7 +22,7 @@
 
 {% block description_source_license %}{% include "_description_source_license.html" %}{% endblock %}
 
-<p>This data as <a href="{{ url_json }}">.json</a>, <a href="{{ url_jsono }}">.jsono</a></p>
+<p>This data as <a href="{{ url_json }}">.json</a></p>
 
 {% include custom_rows_and_columns_templates %}