Fixed FolderExists in FileStore for WindowsStore

The previous implementation tried to get parent folder and see if the
folder asked for exists within that... If the folder passed in as
FolderPath was the LocalStorage folder, then parent will become the root
which the application does not have access to and throw errors.

Another workaround would be to give the app access to the root folder,
but that seems to be not the best way to go about it.
swap IMvxMacViewsContainer for MvxMacViewsContainer as param for RegisterMacViewCreator
AutoLayout goodness for Mac/XamMac
Changes to allow Null literals to be more cleanly parsed #589
Merge branch 'v3.1' of github.com:slodge/MvvmCross into v3.1
null check - fixes #584
Merge pull request #583 from loqu8/mac-cherry

Mac cherry
null check - fixes #584
small refactor - just put the layout constraints in their own method
nuspec changes - 3.1.1 - non-beta
Merge pull request #580 from imason/v3.1

Fixed FolderExists in FileStore for WindowsStore
tidy up catch blocks after #580