Add support for enabling or disabling smoothed lines in a scatter series

I noticed I was getting line smoothing even when a scatter chart's scatter style was set to a non-smoothed one. This change should set the series smoothing defaults based on scatter style of the chart but also allows smoothing to be enabled or disabled on a per-series basis.

Unfortunately this seems to be Excel-specific behaviour and is technically in violation of the Office Open XML standard (see http://msdn.microsoft.com/en-us/library/ff535259(v=office.12).aspx) but since Excel isn't compliant with the standard, I don't see another way to do this and have it work in Excel.

I added tests for default smooth settings and overriding smooth settings.