Comment Outlining

Added OutliningTagger type that creates collapsible regions for blocks of single line comments and blocks of documentation comments. It works well as a complement to the [F# outlining extension](https://visualstudiogallery.msdn.microsoft.com/bec977b8-c9d9-4926-999e-e50c4498df8a).

![fsharp_comment_outlining](https://cloud.githubusercontent.com/assets/680051/5152580/219fb6d2-71c7-11e4-80e3-854e05b97ef3.png)

@vasily-kirichenko  sorry for how sloppy the last PR was
Do not use `_` prefix for private fields. Underscore is used differently in F# then in C#, its main purpose is to mark a symbol as intentionally unused.