VS2013 compatibility.
First version of choice combinator.

--HG--
branch : experimental-choice
Optimize Validation.Match (build regexp only once).
Add confirm piglet for passwords.
merge

--HG--
branch : experimental-choice
Use a memoized function instead of a sequence for Choice cases.

--HG--
branch : experimental-choice
Fix project files target framework version.

--HG--
branch : experimental-choice
Add Select combobox control.

--HG--
branch : experimental-choice
Implement Piglet.Map and Piglet.YieldOption.

--HG--
branch : experimental-choice
Choice: do some stream connections before rendering.

--HG--
branch : experimental-choice
Add YieldFailure() and ReturnFailure().

--HG--
branch : experimental-choice
merge.
Add computation expression for Choices.
merge.
Add Piglet.FlushErrors and Stream.Write.
Add Controls.Link.
Add Reader.Map and fix naming conventions.
Remove alpha suffix.
Add Result.Bind and variants on Reader.Map.
merge.