Use latest psake 4.3.2 for targeting VS 2013 version of MSBuild

Which is needed to build the tests. Updated nuget.targets uses relative
msbuild version
Rm old psake version artifacts
Use SimpleJson 0.38.0 and a preprocessor directive as required for Win Store apps
PCL now targets Windows 8.1 and Windows Phone 8.1 store apps (Universal Apps)

Rm environement properties that aren't available across common
win81/winphone81 framework

Consumer projects can now use this library if they have referenced '.NET
for Windows Store Apps' and one of 'Windows 8.1'/'Windows Phone 8.1'