Adding browserSync for JavaScript file changes

[alex-page]

Addresses #82

Summary of changes:
Updated the gulp file so that the user can make changes to JS without having to recompile.

[geoffp]

Hey, @alex-page -- thanks for the fix! I've taken the gist of this and rolled it into my in-progress branch, aysnc-building and merged it with the new file watcher structure.

Does BrowserSync inject JS or do something special when you say: browserSync.reload('*.js')? If so, do you trust it?

[alex-page]

@geoffp From my understanding and usage of browserSync it doesn't do anything special or inject anything new when adding an additional watch parameter for JavaScript. There is the initial injection for the watching script, which from my understanding just waits for a change to any of the specified paths and files.

That additional line of code is saying:

• Please watch JavaScript files
• If the files change go through your normal process of reloading the page

This allows themer's to easily add small javascript snippets to the patterns which extend functionality without having to reload the page on every change. This is useful for creating an accordion or a dropdown menu pattern.