Fixed Xamarin MvvmCross project files

When code is check out for the first time, nuget packages are checkout out
to a folder called `packages` at the root of the solution.  Previously the
project assumed the nuget packages were in the
`PropertyCross.Core/packages` folder.
Prepared Xamarin MvvmCross package files

Have renamed directory from "xamarin-Mvvmcross" to "xamarinmvvmcross" to

* match the patterns used previously - *remove space characters*
* allow existing build tools to work - *especially for the website*