Adding a basic test suite for spritify

Also adding .editorconfig
Extending spritify to support @media queries

Background images specified in @media queries is not tested yet, and will
probably fail; these background images would have to appear not in the
top-level rule, but in a new media query wrapping that top-level rule.

Supporting sprited images in @media queries is definitely possible, but
more work.
Bumping version to 0.3.0