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)

Link to last PR: https://github.com/DynamoDS/Dynamo/pull/2326
I'm interested in making this even simpler (by getting rid of ```Regex``` altogether), does the following work? We will reject everything other than simple letters and digits (this results in rejection of '-' character also, so we don't need that additional check at the end):

```cs
internal static string NormalizeAsResourceName(string resource)
{
    if (string.IsNullOrWhiteSpace(resource))
        return String.Empty;

    var newText = new StringBuilder(resource.Length);
    foreach (var c in resource.Where(Char.IsLetterOrDigit))
        newText.Append(c);

    return newText.ToString();
}
```