Added a GOES event list utility to sunpy/instr

Following on from #758. Added a utility to sunpy/instr which accepts a TimeRange object as input, and returns a list of GOES flares that occured during that time range. Info for each flare is contained in a dictionary. For example:

    In [4]: from sunpy.instr import goes
    In [5]: from sunpy.time import TimeRange
    In [6]: tran=TimeRange('2011-03-07 00:00','2011-03-07 12:00')

    In [7]: result=goes.get_goes_event_list(tran,goes_class_filter='M1')

    In [8]: result
    Out[8]: 
    [{'end_time': datetime.datetime(2011, 3, 7, 5, 19),
      'event_date': '2011-03-07',
      'goes_class': 'M1.2',
      'goes_location': (0, 0),
      'noaa_active_region': 11164,
      'peak_time': datetime.datetime(2011, 3, 7, 5, 13),
      'start_time': datetime.datetime(2011, 3, 7, 5, 0)},
     {'end_time': datetime.datetime(2011, 3, 7, 7, 56),
      'event_date': '2011-03-07',
      'goes_class': 'M1.5',
      'goes_location': (78, -20),
      'noaa_active_region': 11165,
      'peak_time': datetime.datetime(2011, 3, 7, 7, 54),
      'start_time': datetime.datetime(2011, 3, 7, 7, 49)},
     {'end_time': datetime.datetime(2011, 3, 7, 8, 15),
      'event_date': '2011-03-07',
      'goes_class': 'M1.4',
      'goes_location': (47, 25),
      'noaa_active_region': 11164,
      'peak_time': datetime.datetime(2011, 3, 7, 8, 7),
      'start_time': datetime.datetime(2011, 3, 7, 7, 59)},
     {'end_time': datetime.datetime(2011, 3, 7, 9, 28),
      'event_date': '2011-03-07',
      'goes_class': 'M1.8',
      'goes_location': (50, 23),
      'noaa_active_region': 11164,
      'peak_time': datetime.datetime(2011, 3, 7, 9, 20),
      'start_time': datetime.datetime(2011, 3, 7, 9, 14)}]

The code is basically a wrapper around the HEK functionality.