add license.txt and authors.txt to nuget package
add license.txt and authors.txt to nuget package

alexeymarkov/RxCpp,Reactive-Extensions/RxCpp,alexeymarkov/RxCpp,ReactiveX/RxCpp,alexeymarkov/RxCpp,Reactive-Extensions/RxCpp,ReactiveX/RxCpp,Reactive-Extensions/RxCpp,ReactiveX/RxCpp