Introduce ReviewStatus enum and update RequestReview method

[jamesmontemagno]

Fixes #49 A new enumeration ReviewStatus has been introduced in the Plugin.StoreReview namespace, including values Succeeded, Error, CanceledByUser, NetworkError, and Unknown. The RequestReview(bool testMode) method in multiple files has been modified to return a Task<ReviewStatus> instead of a Task. This method has been updated in StoreReviewImplementation.android.cs to return either ReviewStatus.Succeeded or ReviewStatus.Error based on the status variable, in StoreReviewImplementation.apple.cs to return ReviewStatus.Unknown, and in StoreReviewImplementation.uwp.cs to use the StoreContext.GetDefault().RequestRateAndReviewAppAsync() method and return a ReviewStatus based on the result. Additional using directives have been added to the StoreReviewImplementation.uwp.cs file.###