Add Scan/Reduce with Generic Accumulator Type

[mairbek]

There should be support for generic reducers, for example to implement #32.

Right now Scan restricts accumulator to be the same type as an observable scan(Observable<T> sequence, T initialValue, Func2<T, T, T> accumulator).

This should be changed to scan(Observable<T> sequence, A initialValue, Func2<A, T, A> accumulator).

.NET supports this type of reducers http://msdn.microsoft.com/en-us/library/hh212007(v=vs.103).aspx

[jmhofer]

I already fixed this some time back. scan and reduce are able to use accumulators of a type other than the one of Observable now. Imho this issue can be closed.

[benjchristensen]

Great, thanks @jmhofer

@mairbek I'm going to close this, if you feel otherwise about the status then please re-open.