Little Improvements

Help text for create pitstops form
More precise error messages for create pitstops form
Fixed bu in statistics page when only one pitstop was present

app/rollerverbrauch/templates/statistics.html

@@ -82,7 +82,7 @@
                             {% endif %}
                         </div>
                        <div class="tab-pane " id="v{{vehicle.id}}_c3">
-                            {% if vehicle.pitstop_count > 0 %}
+                            {% if vehicle.pitstop_count > 1 %}
     					        <div id="averageUsageDiv{{vehicle.id}}" style="width:100%; height:500px;"></div>
                                 <script type="text/javascript">
                                     {{ chartScript('averageUsageDiv'+vehicle.id|str, vehicle.average_litres, 'l/100 km') }}