Migrate PowerTip package from jQuery plugins to npm.

[stevenbenner]

The jQuery team have deprecated the jQuery Plugin Registry in favor of npm. The plugin registry is already locked and read only.

The following steps need to be completed:

• [x] Remove jQuery Plugin Registry webhook from repository.
• [x] Any artifacts specific to the jQuery plugin registry should be removed.
• [x] Update package.json to include jQuery plugin keywords (see: npm blog post).
• [x] Set up npm/Grunt build/prepublish/package process to include only built resources necessary for developers implementing PowerTip (no gruntfile, unbuilt source, etc.).
• [x] Attempt to get ownership of the jquery-powertip npm package (@stefanocudini). If successful update name field in package.json. Related to pull request #126.
• [x] Update release process document to use npm.

This will revert #58.

This would also be a good time to add browserify support (See: #130).

[stevenbenner]

I have decided not to hook up an npm prepublish script for this due to npm/npm#10074. Since prepublish would run the Grunt build:npm task on local npm install it would clutter the Travis CI output and confuse any devs who end up seeing unexpected Grunt stuff happening on npm install.

If/when npm adds support for a lifecycle script that is only run before the npm publish command then I should add the Grunt build:npm task there.

Until that point, just make sure to run grunt build:npm before npm publish.

[stevenbenner]

Published to npm as version 1.3.0-rc.1 on the beta distribution tag.

Available for testing by running npm install jquery-powertip@beta.