Support for `"display=none"` #66

Repro case/discussion at Issue #66 

I know nothing about the code base or design, just tried to make it look like the other fields.

I gave this library a complete stress test with [a very complicated set of symbols here] ( https://github.com/Esri/joint-military-symbology-xml/tree/master/svg )

And the library did great, I *think* this was the only change I had to make (so far) to support this entire set. 

Thanks very much for taking over the maintenance of this library, it really helped me on this project (the codeplex version gacked on these symbols). My project that uses your fork [is here](https://github.com/csmoore/MyMiscellanea/tree/master/CSharp/Testing2525D)