Add Bower manifest

This adds a [Bower](https://github.com/bower/bower) manifest as described in the "Defining a package" part of the README.

Using bower, other projects are then able to configure Sneaker as a dependency, and after installing it, have:

```
myproject % tree bower_components
bower_components
└── sneaker
    ├── LICENSE
    ├── README.md
    ├── bower.json
    ├── sneaker-matchers.coffee
    ├── sneaker-matchers.js
    ├── sneaker.coffee
    └── sneaker.js
```

If this PR is merged, the package should be registered with bower so it can be found in the [registry](http://sindresorhus.com/bower-components/):

```
% bower register sneaker https://github.com/livingsocial/sneaker.git
```