This function uses jQuery getScript() to load external JavaScript files, which are printed at the footer of the HTML source. You use it by filtering nesn_load_external_script().

load-external-script.php

/**
 * Hookable filter which loads external scripts in the footer. See the example below
 *
 * <code>
 * filter_nesn_load_external_script( $params ) {
 * 	$params['unique_id']['url'] = 'http://path.to/resource.js';
 *	$params['unique_id']['callback'] = 'callMeAfterScriptLoads';
 *	return $params;
 * }
 *
 * add_filter( 'nesn_load_external_script', $func, 10, 1 );
 * </code>
 * @uses apply_filters, esc_url
 * @action wp_footer
 */
function nesn_external_script_loader() {
?>
<script type="text/javascript">
	jQuery(document).ready(function($) {
		<?php
		$scripts = array();
		$scripts = apply_filters( 'nesn_load_external_script', $scripts );

		if ( ! empty( $scripts ) ) : ?>
		<?php foreach( $scripts as $script ) : ?>
		<?php if ( ! isset( $script['url'] ) ) { continue; } ?>
		jQuery.getScript("<?php echo esc_url( $script['url'], array('http', 'https'), 'not for display' ); ?>"<?php echo ( empty ( $script['callback'] ) ) ? '' :  ', '. $script['callback']; ?>);
		<?php endforeach; ?>
		<?php endif; ?>
	});
</script>
<?php
}

add_action( 'wp_footer', 'nesn_external_script_loader', 100, 0 );