porting to rxjava2

thanks for your good job, i have used this lib in many project.

rxjava2 is stable now, would you porting this lib to it?

i just have a try: https://github.com/skyLiao/rxjava-priority-scheduler/tree/rxjava2,

if you can make a branch, then i can contribute to it.

it's my first time to push code to github, any advise would be ok.