Remove covariance of scan/reduce since consumer/producer are the same <T, T, T>

See https://github.com/Netflix/RxJava/issues/360#issuecomment-24203016
Simplify reduce/scan generics

Updates based on discussion at https://github.com/Netflix/RxJava/pull/369#issuecomment-24255958
Merge pull request #369 from benjchristensen/reduce-and-scan-covariance

Remove covariance of scan/reduce