Xmodem

* bugfix: stall on some kinds of error in `recv()`, PR 56
* bugfix: sequence number miscalculation in `send()`, PR 52
* enhancement: callback function added for `recv()` method, PR 53
* bugfix: receiving empty file and stall condition in `recv()`, PR 50
* bugfix: callback is now called for some kinds of errors and some CLI fixes, 8a798e8b
* bugfix: remove DepreactionWarning for `logging.warn()`, PR 49

- bugfix: Remove bogus assert False code in recv() that resulted in AssertionError introduced in version 0.4.0 commit-id 9b03fc20, PR 29.

- bugfix: Large file transfers in `send()` were more likely to fail for
small values of `retry`: This value should be the maximum failures per
block transfer as documented, but was improperly implemented as the number
of failures allowed for the total duration of the transfer, `PR 21
<https://github.com/tehmaze/xmodem/pull/21>`_.
- bugfix: `send(retry=n)` and `recv(retry=n)` would not retry `n-1`
times, rather than `n` times as documented, causing 'retry=1' to never
retry, for example.