Test for #8

Demonstrates that you can't use a value from an enum defined within a
message as a default value for an optional field in that message.
Fix for #8 (enums as default values)

Names for default values now get wrapped in a Reference instance which
is treated specially in codegen and allows for proper namespacing of
enum values as defaults.
Merge pull request #20 from dowski/master

Fix for enum values as defaults