Work on windows

`asset-rack` didn't work when I cloned it on my work pc running windows. So here a fix to make it cross-platform and a few other changes.

* switched to `express` for testing since we don't need `socket.io`

* changed `package.json` scripts, npm prepends `./node_modules/.bin` to `PATH` so this works cross-platform
   * I have not changed the `postpublish` script because I don't know a cross platform way to delete a folder

* all tests are more consistent with each other
   * use beforeEach to setup `app`
   * use `#{fixturesDir}` in all tests

* moved the `DynamicAsset` test a bit.

* use `.skip` to disable the `SassAsset` tests, better than commenting out.

* added `lib` and `test` to `.npmignore`, no need all that as a dependency right?

* changed `coffee-script` into a `devDependency` for the same reason as above