Update MvxColor.cs

Fixing the output of MvxColor.ToString() to match the description (argb) - Issue #575
Allow creation of MvxViewModelLoader to be overridden.
Removed comment.
Merge pull request #579 from Blewzman/EnableCreateMvxViewModelLoader

Allow creation of MvxViewModelLoader to be overridden.
Merge pull request #576 from iankerr/patch-1

Update MvxColor.cs