RIP Akavache Portable lib

Similar to [the Splat PR](https://github.com/paulcbetts/splat/pull/29), Akavache is now getting rid of the explicit Akavache.Portable.dll - this does **not** mean that Akavache isn't Portable! Instead, there is now a portable version of Akavache.dll. 

This means, that in your PLib, you should reference `Portable-Net45+NetCore45+WP8\Akavache.dll`, but in your app, you reference `Net45\Akavache.dll` (*not* have a reference to both!). If you do this, Everything Just Works.