Warnings fixes and some minor improvements

Hi,

First of all thanks for this great plugin, it made really easy to provide a global search to our site.
I tend to run always with debug on which resulted in some warnings so I fixed them.
Also I needed to search custom post types, change the order of the results and provide a custom screen to show them.
To make all this possible I made some modifications which I shared on a pull request on this repo mirror: https://github.com/wp-plugins/multisite-global-search/pull/1

Hope it helps and if you want me to fix/change something so you can incorporate it just contact me.

Thanks,
Guillermo

https://wordpress.org/plugins/multisite-global-search/