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
Hi @aosyatnik, some minor things:

1. Need the ```foreach``` to be more readable.
2. Comparison with ```.``` and ```-``` are faster than ```IsLetterOrDigit```, put them in front.
3. You have called ```newText.ToString``` two times.

Try this:

```cs
var query = resource.Where(c =>
{
    if (c == '.' || (c == '-'))
        return true;
        
    return Char.IsLetterOrDigit(c);
});

foreach (var c in query)
    newText.Append(c);

var result = newText.ToString();
return ((result == "-") ? string.Empty : result);
```