Structure view implementation

Hi, was hoping to contribute to your IntelliJ plugin.

I thought I would start with a relatively small feature, so decided to implement a structure view for the Rust language. For the most part the change is not too big, but I did add some initial classes for using as mixins, as I figure these will come in handy in the future. 

It still desperately needs some icons that are actually relevant to actual types, but the Platform ones will serve  at least temporarily.

Let me know if there is anything that needs adjusting in my PR. I have been experimenting locally with autocompletion and some basic type checking, and hope to contribute again :).

![structureview](https://cloud.githubusercontent.com/assets/555736/4762294/29c5a840-5b00-11e4-912d-950ebdf92b02.png)