Remove dependence on removed Scheduler#parallelism method

Addresses #2. @MercurieVV, will you confirm this still fits for your use case? Note that it's a breaking change, with a removal of `new PriorityScheduler(ExecutorService)` since there is no longer an implicit `parallelism`, and the switch to static factories instead of public constructors. Assuming that's ok, I'll upgrade and push `0.2` to Maven Central.