Add bower support.

Today I was surprised to discover that there was no [bower](http://bower.io/) package for csslint. I wanted to use csslint on the client in one of my apps, and it would be great to not have to manage it manually.

The only thing you need to add for bower to work is a `bower.json` file in the project root. This pull request adds that. I've also been pretty liberal with the ignores, because presumably anyone wanting to use this on the client only needs the `release/csslint.js` file.

The only unresolved issue I see is how to automate the bumping of the version in `bower.json` for releases. I looked through the Gruntfile to hook into that logic, but it doesn't look like you're currently doing any automated version bumping other than for the CHANGELOG (for instance, the npm package is currently not up-to-date). Anyway, I figured that issue could be solved in a separate pull request, which I'd be happy to help with.

If/when this pull request is merged you can simply run the following command to publish csslint to the bower repo. Note that you'll have to do a release first as bower will look for a specific tag corresponding to a specific version number.

```sh
# install bower globally
npm install -g bower

# register csslint
bower register csslint https://github.com/stubbornella/csslint
```

Let me know if you have any questions or need me to change anything.