Deprecate Gruntfile.js in favor of bootstrap.js

It is non-standard for a Grunt plugin to have a Gruntfile.js that isn't used to configure grunt for development of the plugin. Let's go ahead and use a different file (bootstrap.js) so we can later remove the Gruntfile.js or re-purpose to grunt our grunt.

This change places an error message at the top of output for projects that don't adjust their require statement, but should not interfere with the normal operation of grunt beyond that.