Added support for constant attribute expressions

Hello!

First of all, thank you for a great library! I'm trying to incorporate it into our company's internal tool and I really need suport for constant expressions in attributes, like this one:

[ContractNamespace(Const.Test)]

I've made some changes to resolve constant values upon DOM construction, but it's not scalable and syntax builder doesn't work correctly. Since I only need to parse .cs sources, this patch will work for me for the time being. But it would be really great if you were to add this feature in one of the next versions.

Of course, I don't expect this pull request to be accepted since it breaks syntax builder, it's just an easy way to explain what I mean.