added method to allow for passing of callback URL when getting Request Token
added method to allow for passing of callback URL when getting Request Token

aarondcoleman/Fitbit.NET,AlexGhiondea/Fitbit.NET,WestDiscGolf/Fitbit.NET,amammay/Fitbit.NET,WestDiscGolf/Fitbit.NET,amammay/Fitbit.NET,WestDiscGolf/Fitbit.NET,aarondcoleman/Fitbit.NET,amammay/Fitbit.NET,aarondcoleman/Fitbit.NET,AlexGhiondea/Fitbit.NET,AlexGhiondea/Fitbit.NET