Enum values do not evaluate to equal

This pull request fixes an issue where .NET enum values don't evaluate to equal in Lua scripts. It also fixes two typos in README.md.

Accessing enum values like this worked correctly in NLua 1.3:

```
vala = SomeEnum.ValueA
if vala == SomeEnum.ValueA then
    -- will be executed
end
```

This does not work correctly in NLua 1.3.1. (The equality evaluates to false.) I'm pretty sure this is a side effect of #85. To get that to work again, I just exempted Enums from the fix for #85. I think there may still be an issue, since structs have a default equality operator (as I recall,) and I don't think that will be correctly used in NLua.

I had some trouble getting nunit set up, so it's possible my test case doesn't work.

Isaac