Include WP's plugin.php to fix error on login

I started getting this error recently... 

```
Fatal error: Call to undefined function is_plugin_active() in /home/sofsecur/public_html/amga.com/wp-content/plugins/civi_member_sync/civi_member_sync.php on line 43
```

According to the `is_plugin_active()` function's [documentation](http://codex.wordpress.org/Function_Reference/is_plugin_active), WP's plugin.php much be included before the function is available on the frontend. I'm not sure why this error wasn't occurring for me in the past but the commit in this PR takes of the problem. 

Great work on continuing the development for this plugin! :+1: :+1: