Extracted names of the days of the week to a variable

This is useful for translating the user graphic interface. 

In my application i extend the original renderer and override the ``monthNames`` variable with my translations. To override the day names i have to override the ``monthTitle`` method too changing only the month names. 

This pull request extracts these day names to a variable so that translating the default renderer can be done by simply extending the original one and setting new values for the months and days arrays.