Add references to Oxyplot_NET40 instead of Oxyplot (Portable)
Fix wrong guids referencing
Replace dependency Oxyplot (Portable) by Oxyplot_NET40 in the OxyPlot.GetSharp project.
Merge pull request #283 from thomsonreuters/Remove.PCL.Dependencies

Oxyplot.Core nuget packages still refers to PCL target framework in NET40 folder.