Fixed tuple indexing bug in youtube db load function

There is a bug in the youtube database interface. In particular, running this simple script:

import bob.db.youtube
from bob.bio.video.database.youtube import YoutubeBioFile

youtube_directory = '/idiap/resource/database/YouTubeFaces/aligned_images_DB/'
youtube_extension = ".jpg"

youtube = bob.db.youtube.Database()

files = youtube.objects()
file_num = 0
f = files[file_num]

youtube_bio_file = YoutubeBioFile(f)
frame_container = youtube_bio_file.load(directory=youtube_directory, extension=youtube_extension)
print ( len(frame_container) )

Gives the following error (for the master branch):

Traceback (most recent call last):
  File "/idiap/user/onikisins/Projects/ODIN/Python/scripts/youtube_db/youtube_db_test_script.py", line 40, in <module>
    youtube_bio_file.load(directory=youtube_directory, extension=youtube_extension)
  File "/remote/idiap.svm/user.active/onikisins/Projects/ODIN/Python/experiment_packages/pad_using_local_conda_env_bob27/src/bob.bio.video/bob/bio/video/database/youtube.py", line 41, in load
    file_name = os.path.join(self.make_path(directory, ''), f)
  File "/idiap/user/onikisins/Python/miniconda2/envs/bob-2.7.0-py27_pad_spyder_0/lib/python2.7/posixpath.py", line 68, in join
    if b.startswith('/'):
AttributeError: 'tuple' object has no attribute 'startswith'

This merge request fixes it. The output of the above code is 20, as expected. Can someone, please, review and merge it if OK? Thanks!