Enums should not be (badly) emulated with classes in C# but should
use native C# enums instead. Java implementation made an explicit
choice not to use the (more complex) Java enums, but C# enums are
just light-weight syntactic coating over integral types.

Fixes issue #171.

Change-Id: I9f4d6ba5324400a1e52982e49b58603cb7d7cca7