Turn the plugin into a gem
Don't override ApplicationHelper#javascript_heads anymore

Now that core only includes a bundle for frameworks
(jquery+jquery-ui_ujs) and one file for the application it doesn't make
much sense anymore to concatenate those assets. Previously it was making
6 requests with Prototype, which was worse..
Remove all_core.js.erb (see previous commit)
Adapt plugin to Redmine 2.1.0+
Bump to v0.1.0
Update README