* Thumbnails for albums covers will now be generated on the fly (see notes)

Because thumbnails get sent in full resolution, the page slows down enormously. This code generates a thumbnail on-the-fly (and saves it in memory (/tmp)) and transmits that.

Notes:
* I haven't tested the `x-accel-redirect' implementation, it should work as long as Nginx can access the files located in  /tmp/ 
* Also, pillow is now a requirement, so don't forget to `pip install pillow'