Use Bower instead of Git submodules

[jiehanzheng]

There are a few advantages of using Bower rather than Git submodules. The most important ones include:

• Potentially faster when downloading dependencies, and taking up less space on disk (it removes unnecessary files);
• Ability to specify a version range and receive minor updates without having to explicitly update version number (although this requires semver to work; three.js doesn't use it yet);
• Handles nested dependencies well. Currently, perhaps to prevent loading jQuery twice, we are using w2ui's included jQuery, and if some day w2ui stops depending on jQuery, we have to set up a jQuery submodule again. Bower solves this by resolving all nested dependencies and flatten them--this way we can correctly specify jQuery as this project's dependency without having to worry about having two copies of jQuery;
• Most open source front-end projects I've seen use Bower and many front-end devs are probably more comfortable with it than Git submodules.