Initialize StoreSeries._initialRendering to true in constructor

 StoreSeries._initialRendering should be initialized to true in the constructor or else charts may not draw correctly.

If you are using one or more StoreSeries which use asynchronous stores such as a JsonRest the chart will not scale properly initially. When the first async store is ready with data it triggers StoreSeries._pushDataChanges. The call to chart.updateSeries in this function should set the offsets parameter to true to make sure the the offsets of the chart are recomputed.   

Note that in the old DataSeries module that _initialRendering is correctly set to true in the constructor.

AllenS