Adding support for custom content directory

Adding support for custom content directory using the WordPress plugins_url() method.  

I changed the defined global variable (AMD_ZLRECIPE_PLUGIN_DIRECTORY) to use plugins_url when building up the plugin directory path.  

The WP site url (site_url()) is pulled in later in the zlrecipe.php file, set as a variable, and then used several times to build up the path to the wp-content directory.  I followed a similar pattern of defining an additional variable for the plugins url ($pluginsurl), mostly because I didn't want to break anything. :-)  You might be able to use the previously defined global variable instead, but not sure.

So this works for me when using my custom content directory.  I didn't notice any issues in the Chrome dev tools and the basic functionality of the plugin looks like it's working.  I haven't tried it out in an instance of WP using the default wp-content/plugins directory, so you may want to check that out. 

If you have any questions or feedback, let me know.  Glad to help!