Minify CSS before writing the bundles.

[m-mujica]

Closes #88.

Instead of minifying each css node individually, this PR changes steal-tools to minify the concatenated bundle before writing. I tested this branch in my current app and got some interesting numbers:

• before:
  652K dist/bundles/bundle-a.css
  268K dist/bundles/bundle-b.css
  192K dist/bundles/bundle-a-b.css
• after
  440K dist/bundles/bundle-a.css
  196K dist/bundles/bundle-b.css
  192K dist/bundles/bundle-a-b.css

There is some noise in the write_bundles file b/c I made some changes to the indentation, here is the interesting diff 8c3da0e.

[m-mujica]

@matthewp should I send this PR against minor instead? Not sure why the build failed.

[m-mujica]

The build was failing due to a change in socket.io that socketio/socket.io#2368 broke feathers/testee, a new version of testee fixed the problem.

I also noticed that my main css bundle was quite big, I was able to reduce its size by using less reference imports bringing it down to 276K minified, with this branch the same bundle minified is 272K; which is less of a dramatic improvement, but :itissomething:

[matthewp]

Went over it, looks good. Trying to think of when this should go it. Is it a breaking change? Trying to think of a scenario where your app would behave differently because of the non-duplicate selectors.

[matthewp]

Putting it under the 1.0.0 milestone for now, could go in sooner.

[matthewp]

This will go on 0.14.0

[matthewp]

This is merged into minor and part of 0.14.0-pre.0. Closing.