Use true timestamp to avoid "Invalid timestamp"

With the original version i got :

python /usr/lib/python2.6/site-packages/pushover.py --token "NU2LC5g2G9jgVR9eHVLYAUmkFuPeky" --client "FWhJFb5wgV35bmzmaxeV976K1WqaFE" "Hello"
Traceback (most recent call last):
  File "/usr/lib/python2.6/site-packages/pushover.py", line 227, in <module>
    url_title=args.url_title, timestamp=True)
  File "/usr/lib/python2.6/site-packages/pushover.py", line 207, in send_message
    return MessageRequest(payload)
  File "/usr/lib/python2.6/site-packages/pushover.py", line 104, in __init__
    Request.__init__(self, "post", MESSAGE_URL, payload)
  File "/usr/lib/python2.6/site-packages/pushover.py", line 86, in __init__
    raise RequestError(self.answer["errors"])
__main__.RequestError:
==> timestamp is invalid


By casting time.time() the error disappears
Merge pull request #1 from Crupuk/patch-1

Use true timestamp to avoid "Invalid timestamp"