Adds broken unit test for Builder properties.

This doesn't compile until we fix issue #9.
Prototype of fix for builder properties.

No actual T4 changes made. Just hacked at its output.
Merge remote-tracking branch 'origin/master' into PropertiesAreBuilders9

Conflicts:
	ImmutableObjectGraph.Tests/Person.generated.cs
	ImmutableObjectGraph.Tests/PersonBuilderTests.cs
Adds test for setting Builder properties to null.
Removes extra field for each Builder property.
T4 template now generates the required source.
Moves method position.
Builders' properties for other immutable objects are now Builders as well.

Closes #9