Use argparse module instead of the deprecated optparse module. Check if tzinfo is present in the timestamp. Some PEP8

* [argparse](http://docs.python.org/2/library/optparse.html) is deprecated since version 2.7.
* Check if tzinfo is present in the timestamp to prevent this error : 

```
./poll_client.py --begin_timestamp "2009-08-18T13:52:54"
Traceback (most recent call last):
  File "./poll_client.py", line 50, in <module>
    main()
  File "./poll_client.py", line 39, in main
    inclusive_end_timestamp_label=end_ts)
  File "/home/yohann/.virtualenvs/yeti-test/src/libtaxii/libtaxii/messages.py", line 1370, in __init__
    raise ValueError('exclusive_begin_timestamp_label.tzinfo must not be None')
ValueError: exclusive_begin_timestamp_label.tzinfo must not be None

```
 
* Some PEP8.