Add ProgressbarLock

Takes care that no two progressbars are active at the same time if not in interactive mode.

If @simon.humpohl has an idea how to

• implement this more nicely
• implement this such that using the progressbar in a with block works;

  with tqdm(...) as bar:
      for i in it:
          bar.n = i
          bar.refresh()

I'm all ears. I'm also fine with making tqdm are hard dependency (usuallly installed anyway) and throwing out the _simple_progressbar.

To Do

• fix doctest
• Apply lock for QTconsole only (nested bars work for jupyter notebook, ipython and python)