Implements #275.

file argument is now set to None and defined at instanciation inside __init__() (instead of at class import). This should ease file redirection (as long as sys.stdout is redirected before tqdm instanciation, it should be fine).

TODO:

• Use file=None and set default self.fp=sys.stderr inside init. Same for tqdm.write() (and other methods/submodules?). This will allow users to redirect sys.stderr before instanciating tqdm, without needing to supply the new redirection to tqdm.
• Check that it doesn't impact the stdout redirection scheme described in the readme.
• Add unit test for stdout (or a dummy IO) redirection (2 tests: 1- redirect using wrapper like in README, 2- redirect stdout before instanciating tqdm, this is the new redirection scheme provided by this PR).

TODO in a separate PR resiliency-io-error:

• Wrap any self.fp.write() (tqdm.move() and status_printer?) in try/except block (this should not impact performances as the except block will only get evaluated if necessary). The except block should still display any error but as a non blocking message (using a new TqdmErrorMessage() class?). Something like this:

def move(n):
    try:
        #...
    except IOError as exc:
        if not self.resiliency:
            raise(exc)