Add Additional statements for exporting individual Enum Values See #74

After exposing last enum method in [https://github.com/square/javawriter/pull/42] I have added additional statements which allow individual Enum values to be exposed. 

Now Enums look like this:

```java
public enum FooBarBazEnum {
    @ProtoEnum(1)
    FOO,
    @ProtoEnum(2)
    BAR,
    @ProtoEnum(3)
    BAZ;
    public static final Integer FOO_VALUE = 1;
    public static final Integer BAR_VALUE = 2;
    public static final Integer BAZ_VALUE = 3;
  }
```

This change would require a SNAPSHOT version of JavaWriter and I am not quite sure which tests it's going to break and need fixing. 

Comments? @danrice-square