Add recommended `main` field to bower.json to help build tools find the source.

This makes it possible for build tools such as [Webpack](https://github.com/webpack/webpack) to [resolve the package using bower.json.](https://github.com/webpack/webpack/issues/143#issuecomment-32866443)

[bower.json specification](https://github.com/bower/bower.json-spec#main) recommends adding this field.

Also removed `bower.json` from the ignore list in bower.json,
to prevent it from being removed when the package is installed,
so it can be used for this purpose.