add PrimaryKey struct
Merge branch 'master' into PrimaryKey
PrimaryKey compiles
more on PrimaryKey
even more on PrimaryKey
remove SingleAttribute in favor of SingleAttribute and more on PrimaryKey
PrimaryKey almost finished
Merge branch 'master' into PrimaryKey

Conflicts:
	Signum.Engine/Schema/EntityEvents.cs
	Signum.Engine/Schema/Schema.Basics.cs
	Signum.Engine/Schema/Schema.Save.cs
	Signum.Engine/Schema/SchemaBuilder/SchemaBuilderSettings.cs
	Signum.Web/Operations/OperationClient.cs
	Signum.Windows/Operations/OperationClient.cs
fix Sync with Identity and Default