Fix Pinboard JSON duplicate timestamps error

[bardisty]

If the JSON exported by Pinboard contains duplicate timestamps, Python
returns a TypeError exception:

TypeError: argument of type 'float' is not iterable

This is because time.mktime() returns a floating point number.
Encasing time.mktime() in str() fixes the data type not being
iterable.

time.mktime() has also been encased in int() to remove the
unnecessary decimal value (.0) that gets returned for each time value,
and to keep the script consistent with the other export functions.

Full Error:

$ env CHROME_BINARY=google-chrome-stable ./archive.py ~/tmp/test
[+] [2017-07-01 02:57:35] Starting archive from /home/bah/tmp/test export file.
Traceback (most recent call last):
  File "./archive.py", line 468, in <module>
    create_archive(export_file, service=export_type, resume=resume_from)
  File "./archive.py", line 412, in create_archive
    links = uniquefied_links(links)         # fix duplicate timestamps, returns sorted list
  File "./archive.py", line 283, in uniquefied_links
    link['timestamp'] = next_uniq_timestamp(seen_timestamps, link['timestamp'])
  File "./archive.py", line 251, in next_uniq_timestamp
    if '.' in timestamp:
TypeError: argument of type 'float' is not iterable

JSON test data:

[{"href":"https:\/\/pushover.net\/","description":"Pushover: Simple Notifications for Android, iOS, and Desktop","extended":"","meta":"1e68511234d9390d10b7772c8ccc4b9e","hash":"bb93374ead8a937b18c7c46e13168a7d","time":"2014-06-14T15:51:42Z","shared":"no","toread":"no","tags":"app android"},
{"href":"http:\/\/www.reddit.com\/r\/Android","description":"r\/android","extended":"","meta":"18a973f09c9cc0608c116967b64e0419","hash":"910293f019c2f4bb1a749fb937ba58e3","time":"2014-06-14T15:51:42Z","shared":"no","toread":"no","tags":"reddit android"}]

Output after fix:

$ env CHROME_BINARY=google-chrome-stable ./archive.py ~/tmp/test           
[+] [2017-07-01 04:09:17] Starting archive from /home/bah/tmp/test export file.
[*] [2017-07-01 04:09:17] Created archive index with 2 links.
[*] Checking Dependencies:
/bin/google-chrome-stable
/bin/wget
/bin/curl
[+] [1402786302 (2014-06-14 15:51:42)] "Pushover: Simple Notifications for Android, iOS, and Desktop": pushover.net/
    - Downloading Full Site
    - Printing PDF
      Exception: Exception Failed to print PDF
    - Snapping Screenshot
    - Submitting to archive.org
    - Fetching Favicon
[+] [1402786302.1 (2014-06-14 15:51:42)] "r/android": www.reddit.com/r/Android
    - Downloading Full Site
    - Printing PDF
      Exception: Exception Failed to print PDF
    - Snapping Screenshot
    - Submitting to archive.org
    - Fetching Favicon
[√] [2017-07-01 04:09:39.035009] Archive update complete.

[pirate]

Awesome PR, thanks so much!

Would you mind actually also commiting your test file under examples/pinboard.json. It's a useful testcase to have in the repo.