Add ffpyplayer provider for image

Here's some test data, it seems that ffpyplayer does increasingly better with larger images. So e.g. with gifs where there are lots of small images and not much decompression, it doesn't do a lot better than pil, but it does do a lot better with jpegs.

test code:

```python
from kivy.core.image.img_pil import ImageLoaderPIL
from kivy.core.image.img_ffpyplayer import ImageLoaderFFPy
from functools import partial
import timeit
from os.path import join
import logging
logging.root.setLevel(logging.ERROR)

path = r'C:\kivy-dev'
filenames = ('DSCN3974.jpg', 'DSCN4465.jpg', 'DSCN5081.jpg', 'test.jpeg',
             'mod1.jpg', 'skaters.gif', 'dsdfj.gif', '7.gif', 'Boglio_03.gif')
count = 25
pil = ImageLoaderPIL(join(path, filenames[0]))
ffpy = ImageLoaderFFPy(join(path, filenames[0]))

for filename in filenames:
    p = timeit.Timer(partial(pil.load, join(path, filename))).repeat(1, count)[0] / float(count)
    ff = timeit.Timer(partial(ffpy.load, join(path, filename))).repeat(1, count)[0] / float(count)
    print('{}: pil: {}, ffpy: {}'.format(filename, p, ff))

```

results:
```
DSCN3974.jpg: pil: 0.257700669428, ffpy: 0.0616306165698
DSCN4465.jpg: pil: 0.228653756055, ffpy: 0.0512112003108
DSCN5081.jpg: pil: 0.225296389324, ffpy: 0.0853224963363
test.jpeg: pil: 0.0146633209615, ffpy: 0.00491458966453
mod1.jpg: pil: 0.0517993184583, ffpy: 0.026293465322
skaters.gif: pil: 0.16230622072, ffpy: 0.113907991464
dsdfj.gif: pil: 0.0174292517619, ffpy: 0.0146146053912
7.gif: pil: 0.00897177872192, ffpy: 0.00501039476283
Boglio_03.gif: pil: 0.0675255947681, ffpy: 0.0492512937507
```

test files:
http://i242.photobucket.com/albums/ff231/bour3/anims/skaters.gif
http://i224.photobucket.com/albums/dd216/123draw-images/dsdfj.gif
http://i964.photobucket.com/albums/ae128/shurkeminaka/gif/7.gif
http://www.wired.com/wp-content/uploads/images_blogs/design/2013/09/Boglio_03.gif
http://compression.ca/act/files/DSCN3974.jpg
http://compression.ca/act/files/DSCN4465.jpg
http://compression.ca/act/files/DSCN5081.jpg
https://www.imperialviolet.org/binary/jpeg/test.jpeg
http://www.stevehuffphoto.com/wp-content/uploads/2013/09/mod1.jpg