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
So you'd prefer 

       [<Struct>]
       type Region (startline: int, endline:int, collapsed:string, contents:string) = 
            /// Line number of the first line in the region
            member __.StartLine = startline
            /// Line number of the last line in the region
            member __.EndLine = endline
            /// The text displayed inline when a region is collapsed
            member __.Collapsed = collapsed
            /// The text displayed in the tooltip when a collapsed region is moused over
            member __.Contents = contents

            override x.ToString() = sprintf "[Region {ln %d - ln %d}]" x.StartLine x.EndLine

?