Add Prepend and Append to Rx

[bartdesmet]

To be on par with LINQ to Objects, we should add these.

[mattpodwysocki]

@bartdesmet let's go even further to rename StartWith to Prepend, deprecating StartWith, adding an n-ary approach

[clairernovotny]

Moving to vNext so we can get 4.0 out as per our discussion.

[quinmars]

Is this up for grabs? This looks like a nice task to get hands on.

[akarnokd]

@quinmars I'd think you can start working on this. Perhaps put it into the System.Reactive.Observable.Aliases.QueryLanguage so that you can have fun with its Qbservable generated counterpart :)

[quinmars]

Remains the question if we should choose the n-ary approach or not. Personally I'd would prefer to exactly mirror the IEnumerable/IQueryable counter parts (not variadic) and keep StartsWith() as it is. Once/if the corefx team decides to add an AppendMany() or PrependMany() we can consider to replace StartsWith() with PrependMany().