Add a watch command

Add gulp-watch to the project and add an explicit watch command. include a `watchPath` property in the config which conforms to a gulp-watch glob. If no watchPath property is set it should maybe default to project root with exceptions for common files and directories e.g. `node_modules` `bower_components` `dist` `vendor` `build` `bower.json` `package.json` or invisible files & directories. Documentation should be clear about this and include some tested glob boilerplate.