CssSourceMap: Generate on separate thread

Generate on separate thread.
* Raise notifier after completion.
* Prevent UI freezing.
* Fixes #848.

After reconsidering async/await model, it seems like it wouln't serve the purpose (see this [test commit](https://github.com/am11/WebEssentials2013/commit/037f05841711cccb414b4f851b22c1ee95541a40)). 

The idea really is to render interim result (output from compiler) in the preview window as soon as possible. Once the heavy-lifting, "sourcemap generation", task is completed, sourcemap would raise an event to notify `CssTextViewMargin`, so to activate sourcemap-dependent features (selectivity, go-to-definition).