[WIP] Upgrade Octokit .NETCore library to Windows 8.1

So even though you can open the Octokit solution in VS2013, there's a subtle dependency on VS2012 that was hard to notice, even after changing the target to Windows 8.1. So I've hacked the csproj files and it's now :gem::gem::gem:

![screen shot 2014-03-15 at 5 18 50 pm](https://f.cloud.github.com/assets/359239/2427782/b7edb4a6-ac09-11e3-9ce8-0b4909ebb683.png)

This PR makes it possible to build the project with just VS2013, but is likely to be superseded by a PCL build :soon:

:cactus::cactus::cactus:
**UH-OH SPAGHETTI-O`S**
Because of this fix, I've needed to drop support for Windows Store Apps targeting Win8. 

I've updated the `nuspec` files to target `netcore451` as per [how another package does it](https://github.com/timheuer/callisto/blob/master/Tools/nuget/Callisto/Callisto.nuspec#L25)
:cactus::cactus::cactus:

cc @adamchester to confirm this is the right fix

Resolves #143