Made power rankings get current week by default and week parameter optional

[dtcarls]

Attempting to resolve #31 Please double check functionality, not sure how to thoroughly test.

[dtcarls]

Tested as follows.

$ python3 setup.py install
$ python3
>>> from espnff import League
>>> league_id = 164483
>>> year = 2016
>>> league = League(league_id, year)
>>> league.get_current_week()
18
>>> year = 2017
>>> league = League(league_id, year)
>>> league.get_current_week()
1

[dtcarls]

Update to verify it works correctly going into week 2

$ python3
>>> from espnff import League
>>> league = League(164483, 2017)
>>> league.get_current_week();
2