Make it Pillow compatible

I don't know the reason, but I have to import from PIL or I get a `IOError: cannot identify image file`. I found several reports about this problem in StackOverflow (e.g. http://stackoverflow.com/questions/17626042/trouble-when-open-image-with-pillow-in-python).
Fix some quotes in readme
Using `url_for` to generate Flask-Images URLs instead of a custom function. Use the route `images` (configurable with `IMAGES_NAME`).

e.g. `url_for('images', filename='image.png', width=30, mode='crop')`

I didn't removed the `resized_img_src` behaviour to maintain compatibility.
Merge branch 'pillow'
Merge branch 'readme'
Merge branch 'url_for'
Setup for Travis