MAGN-4275: Create resource file with images for Operators

# Preface
Operators is one of categories, that generates its' nodes directly in Dynamo. Moreover, these nodes have names like `-`,`+`,`>=` etc. That's why we can't generate icon name as usual.
# Solution
Where you should pay attention:
1. Added default assembly DynamoCore. In this assembly we will look for icons for such members like operators and builtin functions.
2. We can't use `QualifiedName`as rule for generating icon title( ==.Small.png Visual Studio do not allow such names for resource). So that, we have to use `Name` property for generating icon's title.
3. And added resource files(small and large).
# Before
![image](https://cloud.githubusercontent.com/assets/8158404/4148152/7fb3fda2-3421-11e4-8d4f-30af9e5cf053.png)
# After
![image](https://cloud.githubusercontent.com/assets/8158404/4148119/26432df6-3421-11e4-9286-935293586248.png)
# Reviewers
@Benglin, please take a look.

Link to YouTrack:
[MAGN-4275 DUI: Create resource file with images for Operators](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4275)
I think the above is a little... fragile. We might not want to check for ```Operators``` category name, what if it is changed to ```Ops```? Then this will break. Hard-coded names in source code are a bad idea.

Okay here's what I could suggest:

1. Write a utility function ```string NormalizeAsResourceName(string name)```.
2. First try ```var name = NormalizeAsResourceName(this.FullCategoryName);```
3. Then test ```if (string.IsNullOrEmpty(name))```, then try ```name = NormalizeAsResourceName(QualifiedName)```.
4. In ```NormalizeAsResourceName``` utility method, we remove characters that is illegal for a file name (e.g. ```=```, ```+```, etc.).
5. If ```Name``` is only made up of invalid characters like that, it will result in an empty name, so the caller falls back to using ```QualifiedName```.
6. Write test cases for this new utility method to test all known scenarios in the library as of today.