Add chart note support

It is saved under /chartinfo/note, and may be non-existant or an empty
tag.

Fixes #28

src/ag-chart.h

@@ -60,6 +60,8 @@ gchar *ag_chart_create_svg(AgChart *chart,
                            gsize   *length,
                            GError  **err);
 GList *ag_chart_get_planets(AgChart *chart);
+void ag_chart_set_note(AgChart *chart, const gchar *note);
+const gchar *ag_chart_get_note(AgChart *chart);
 
 #define AG_CHART_ERROR (ag_chart_error_quark())
 GQuark ag_chart_error_quark(void);