Merge #82: Display Bitcoin timestamps with appropriate precision

44a1951 Display Bitcoin timestamps with appropriate precision (Peter Todd)

Pull request description:

  Previously would display them with precision down to the second, which misrepresents how precise a Bitcoin timestamp can actually be as adversarial miners can get away with creating blocks whose timestamps are inaccurate by multiple hours or more.

  For those who do want a more precise timestamp, the height of the block attesting to the timestamp is now displayed, allowing a manual investigation of it.

  Examples of the new UX:

  ```
  $ ots verify examples/empty.ots
  Assuming target filename is 'examples/empty'
  Success! Bitcoin block 129405 attests existence as of 2011-06-08 EDT
  ```

  ```
  $ git tag -v opentimestamps-client-v0.5.1
  object dcc4549
  type commit
  tag opentimestamps-client-v0.5.1
  tagger Peter Todd <[email protected]> 1513029381 -0500

  Release opentimestamps-client-v0.5.1
  ots: Got 1 attestation(s) from https://finney.calendar.eternitywall.com
  ots: Got 1 attestation(s) from https://bob.btc.calendar.opentimestamps.org
  ots: Success! Bitcoin block 498825 attests existence as of 2017-12-11 EST
  ots: Good timestamp
  gpg: Signature made Mon 11 Dec 2017 04:56:22 PM EST
  gpg:                using RSA key 2481403DA5F091FB
  gpg: Good signature from "Peter Todd <[email protected]>"
  gpg:                 aka "[jpeg image of size 5220]"
  ```

Tree-SHA512: e4e6186b493d0b6ab59994ceaa60da10d13e5464219fb3a45760900b12d09f7d270c387600cc43cef65482c2455401adc8776615e6dcd5a4b5a4d60a8e825405

README.md

@@ -151,11 +151,6 @@ Use the setuptools development mode:
 
 ## Known Issues
 
-* Displaying Bitcoin timestamps down to the second is false precision, and
-  misleading. But rounding off to the nearest day is over-doing it in the other
-  direction. See https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2016-September/013120.html
-  for more information on this issue.
-
 * Need unit tests for the client.
 
 * Git tree re-hashing support fails on certain filenames with invalid unicode

otsclient/cmds.py

@@ -427,8 +427,10 @@ def attestation_key(item):
                 logging.error("Bitcoin verification failed: %s" % str(err))
                 continue
 
-            logging.info("Success! Bitcoin attests data existed as of %s" %
-                         time.strftime('%c %Z', time.localtime(attested_time)))
+            logging.info("Success! Bitcoin block %d attests existence as of %s" %
+                            (attestation.height,
+                             time.strftime('%Y-%m-%d %Z',
+                                          time.localtime(attested_time))))
             good = True
 
             # One Bitcoin attestation is enough