Add color options for reference types, value types and patterns

I made a few changes so that color options now are available in Tools --> Options --> Environment --> Fonts and Colors --> Display items.

There are three defined colors: 'F# User Types', 'F# User Types (Value types)', and 'F# Patterns'. 
In C#, there are 6 colors which are available in Display items under `User Types` prefix. So we probably have a few more colors to put into good use.

Here is current screenshot. Feel free to adjust the colors (I chose different ones for easy testing)

![image](https://f.cloud.github.com/assets/941060/2426825/ecb85cc8-abcc-11e3-9bdf-c43a2b66febc.png)