DUI: icons for class buttons, method buttons, tooltips.

This PR was made to implement Small icons for class buttons & method buttons; Large icons for tooltips.
# Before
![image](https://cloud.githubusercontent.com/assets/8158404/4057010/ea287b00-2dc2-11e4-899b-03e68c08ae5b.png)

# After
![capture](https://cloud.githubusercontent.com/assets/8158404/4061382/81977b10-2df7-11e4-8b18-78f188937443.PNG)

# Pay attention
I know that it's bad, when I change a lot of code, but... You should pay attention on `BrowserInternalElement`. It's thing from which we draw class button(and it had only Name and that's all). I had to add to its' constructor 1 more value `assembly`, so that class button will know in which assembly should it search icon for itself.

# Reviewers

@Benglin, please take a look.

P.S. Large icon have not done yet.
The following logic has quite a bit of common codes and should be refactored (it seems like in your logic, only those ```DLL``` with ```*.resource.dll``` will result in ```LibraryCustomization``` being created, which is different from the original logic):

```cs
XDocument xDocument = null;
Assembly resAssembly = null;

if (ResolveForAssembly(...))
    xDocument = XDocument.Load(customizationPath);

if (ResolveForAssembly(...))
    resAssembly = Assembly.LoadFrom(resourceAssemblyPath);

// We need 'LibraryCustomization' if either one is not 'null'
if (xDocument != null || (resAssembly != null))
{
    var c = new LibraryCustomization(...);
    return c;
}

triedPaths.Add(assemblyPath, false);
return null;
```

This way we don't need two constructors of ```LibraryCustomization```.