Undefined call to add_management_page error

This Error was was received upon activating the plugin. 

Fatal error: Call to undefined function add_management_page() in .../plugins/WP-HTTP-API-Tester-master/includes/admin.php on line 10

After viewing Otto's reply to a similar issue (http://wordpress.org/support/topic/take-2-minutes-and-help-me-out#post-867490) I adjusted the code slightly.

This pull request removes add_management_page from construct and creates a management_page callback. Then hooks into admin_menu.

**Note: This may cause the plugin to not function during GET and POST. I'm still testing to see why I'm not getting any response back using the default GET.