tgext.asyncjob updates

[Alessandro Molina]

tgext.asyncjob always had the limit of being unable to track task progress between multiple processes, I decided to implement various task progress tracking backends to solve this issue. Currently only the Redis backend is provided, but implementing more is really simple.

As asyncjob was actually a really small package I took the chance to write a testsuite and port it to TG2.3 and Python3. I also created a github mirror for the repository so it is automatically tested through travis-ci: https://travis-ci.org/amol-/tgext.asyncjob/builds/7298170

Feel free to test it!

If no major bugs will be reported I'll make a new release next week.