Added support for MetroIconButton

### MetroIconButton

![iconbuttons](https://cloud.githubusercontent.com/assets/574734/2590625/da17a9c4-ba5f-11e3-9e6a-11ac6a962bf4.png)

### Usage

```xml

<Controls:MetroIconButton Width="100"
                          Margin="0,10,0,0"
                          Content="BUTTON"
                          Icon="{DynamicResource appbar_anchor}"/>
```

Has support for all the icons packaged with [MahApps.Metro.Resources](http://www.nuget.org/packages/MahApps.Metro.Resources). 

### Structure of Control

`MetroIconButton` uses a `ContentPresenter` placed at the left of the button's default `ContentPresenter` to display the icons. 

```xml
<ContentPresenter Width="{TemplateBinding IconWidth}"
                  Height="{TemplateBinding IconHeight}"
                  Margin="{TemplateBinding IconMargin}"
                  HorizontalAlignment="Center"
                  VerticalAlignment="Center"
                  Content="{TemplateBinding Icon}"
                  RecognizesAccessKey="True"
                  RenderTransform="{TemplateBinding IconRenderTransform}" />

<ContentPresenter x:Name="contentPresenter"
                  Grid.Column="1"
                  Margin="{TemplateBinding Padding}"
                  HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                  VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                  Content="{TemplateBinding Content,
                                            Converter={StaticResource ToUpperConverter}}"
                  ContentTemplate="{TemplateBinding ContentTemplate}"
                  RecognizesAccessKey="True" />
```

### Customization

The control comes with a few DependencyProperties for users to customize the position of the icon's `ContentPresenter`. This is necessary as icons are sometimes too large or do not align correctly. Listed below are the DependencyProperties that come built in with the `MetroIconButton`. 

+   `Icon`:   This property is bound to the `Content` property of a `ContentPresenter`. You can either use the icons packaged with `MahApps.Metro.Resources` (`{Dynamicresource appbar_alien}`) or set it to any FrameworkElement of your choice.
+   `IconRenderTransform`:    Bound to the `RenderTransform` property of the `ContentPresenter`. Used this to adjust the size of the icon.
+   `IconWidth`:  Bound to the `Width` property of the ContentPresenter. Use this to adjust the icon's container space.
+   `IconHeight`: Bound to the `Height` property of the ContentPresenter. Use this to adjust the icon's container space.
+   `IconMargin`: Bound to the `Margin` property of the ContentPresenter. Use this to adjust the icon's position relative to the button's text

> Note, the `ContentPresenter` mentioned in the documentation above refers to the `ContentPresenter` responsible for displaying the image