Merge in core split work
Set all projects to .NET 4.0
ShouldDisplayPreviewCore is a property, not a method
Revert changs in CS.props file
Fix changes to RevitNodesUI.csproj
Remove .NET 4.5 framework version prop from test csproj files
Merge pull request #51 from pboyer/DynamoCoreSplitRevit2016

Dynamo core split : Revit2016