Handlebars Compiler

Addition of a compiler for handlebars templates.

Defaults to in place, and version 1.3.0.
You shouldn't need this file; compilations should be invoked by `EditorCompilerInvoker`.
That isn't working for you because you never create an instance of it (we usually create an instance in the [margin](https://github.com/madskristensen/WebEssentials2013/blob/master/EditorExtensions/Shared/Margins/MarginBase.cs#L187), but preview margins don't make sense for Handlebars).

Instead, you should create an `IWpfTextViewConnectionListener` and write

```csharp
textView.Properties.GetOrCreateSingletonProperty(() =>
    Mef.GetImport<ICompilationNotifierProvider>(textView.TextBuffer.ContentType)
    .GetCompilationNotifier(document));
```

And then dispose it (from `GetProperty<ICompilationNotifier>`) in `SubjectBuffersDisconnected()` (otherwise, errors will never vanish)