Better integration with AiOSEO plugin

Hi,

I added a little tweak to integrate Category SEO Meta Tags with All in One SEO using filters/actions API, i.e. you no longer need to modify AiOSEO core files in order to get category titles working.

Also, got rid of several PHP notices.

TODO: 
1. Better options handling. Currently it is possible to have empty title formats by default so nothing works.
2. Better taxonomy handling. If a taxonomy is not selected, then CSMT form should not be displayed to avoid confusion, because in this case options are not saved.
3. There are few more PHP noticed that should be fixed.