Remove node-canvas as dependency

feat: sprite-height/width functions

Added sprite-height and sprite-width functions for being able to use sprite dimensions in calculations like:

```scss
.blah {
    margin-top: sprite-height($my-image) + 10px;
}
```

Update readme

* add syntax highlighting to the code blocks
* use svg badges instead of png badges
* update references

Conflicts:
	lib/css-sprite.js

Merge branch 'binary_tree_packing' of https://github.com/calebrash/css-sprite into calebrash-binary_tree_packing

Conflicts:
	package.json

place media query inside of class for better extending