Add WinPhone SignInDialog
Remove useless file
Merge branch 'v.next' into dom/issue65_SignInDialogWinPhone
Accommodate to Credential API changes.
Add WinPhoneChallengeHandler.
Support for Credential storage.
Merge branch 'v.next' into dom/issue65_SignInDialogWinPhone

Conflicts:
	src/WinPhone/Esri.ArcGISRuntime.Toolkit/Themes/Generic.xaml
Implement IChallengeHandler interface (now that it's checked in the API)
Rename PhoneChallengeHandler
Merge branch 'v.next' into dom/issue65_SignInDialogWinPhone

Conflicts:
	src/WinPhone/Esri.ArcGISRuntime.Toolkit/Esri.ArcGISRuntime.Toolkit.WindowsPhone.csproj
Merge pull request #82 from Esri/dom/issue65_SignInDialogWinPhone

SignInDialog for WinPhone