TINKERPOP-1552 C# GLV: Generics and enum generation

[jorgebay]

Added type safety to traversal interface. For example:

Vertex vertex = g.V().Next();
// Compile time error
Edge edge = g.V().Next();

As C# generics is more strict than in Java, some methods require the user to specify the expected type:

// Specify the type of the value of the property "age"
IList<int> userAgeList = g.V().HasLabel("user").Values<int>("age").ToList();

Additionally, I've changed the letter case of the enums generated in C# to match the C# naming convention, ie: g.V().Values<int>("age").Order().By(Order.Decr)

[jorgebay]

cc @FlorianHockmann