Added an overload to the Sample operator to take another observable t…

[svermeulen]

…o use to determine the sample interval. This is just a more generic version of SampleFrame. To minimize duplication I also changed SampleFrame to be implemented based on this more generic version.

[neuecc]

In performance reason, Sample will be replaced optimized code #165
so should not replace SampleFrameObservable to SampleObservableObseravble.

This is missing operator of Rx.NET Sample<T, TSample>(IObservable<T> source, IObservable<TSample> sampler). #105
Yes, this is necessary method, thank you!
But it should locate in Operators/Sample https://github.com/neuecc/UniRx/blob/master/Assets/Plugins/UniRx/Scripts/Operators/Sample.cs

[svermeulen]

Ok I'll submit it again with those changes.

[svermeulen]

Did I understand those changes correctly?

[neuecc]

Thank you!
almost okay.
I'll merge and fix after it.