Add "main" field to bower.json

[donv]

This should fix the following message from rails-assets-tether:

This component doesn't define main assets in bower.json.
Please open new pull request in component's repository:
https://github.com/harvesthq/chosen

[tjschuck]

Chosen does not support command-line bower install chosen installation. This is because the repo does not contain the compiled sources, and bower does not currently support adding a build step.

You can specify that you'd like to use the release ZIP file in your own project's bower.json file.

See #1509, #1472, #1333, and #1316 for more info.

[koenpunt]

@tjschuck We should probably add common issues to the README. Or add them in the wiki and add a link to that.

[pfiller]

bower install chosen

As of 1.4.2, Chosen supports bower installs. Read the 1.4.2 release notes for details.