Use sprite sheet for most of our images

In the process of decreasing http requests to decrease load time, a sprite sheet was created to include most of the images mist.io uses. The workflow to add an image gets a little bit more complicated, but the payoff is worth the trouble (same with the pre-compiled templates)

Workflow
-------------

- For adding images on the sprite sheet
1) place them inside *src/mist/io/static/images/sprite-images*
2) run ```./bin/python scripts/sprite_generator.py```
**FYTK**: the script uses the configuration file .sprite_generator which exists on the top level directory
3) a css class selector will be generated with the name of the image file.
**FYTK**: the generated css file and the sprite sheet are outputed into *src/mist/io/static/images/sprite-build* . The css file is already imported into main.css
   
- For adding images NOT on the sprite sheet AND preloading them
1) place the image wherever you want (ideally in src/mist/io/static/images)
2) add it on the images array in app.js, it's in the "preloadImages" function
3) use your own css

Payoff
----------
Well the numbers speak for themselves. Here is the average image preloading time in seconds, measured on our dev server using JS_BUILD:

- Before
   - cached: 1848
   - uncached: 541
- After
   - cached: 760 
   - uncached: 257

*It's basically down to half...*

Setup
--------
To setup the sprite generation script we need 3 things:
1) the actual script
2) it's configuration file
3) it's dependencies

The script is a variation I made (https://github.com/gtsop/sprite-generator/tree/config_subdir_discovery) on this repo (https://github.com/dennisschaaf/sprite-generator)

The configuration must be named *.sprite_generator* and must exist on the same directory as the script or higher.

The only dependency is Pillow. However, we need to install libjpeg for jpeg support:
```sudo apt-get install libjpeg-dev``` for linux
```brew install libjpeg``` for osx
and then:
```pip install -I pillow```

As the configuration file and the script will be into the source code, the only thing that we really need to do IF we need to run the sprite_generator.py, is to install the above dependencies

EOF