Directory name agnostic plugin dependency check, Fixes compatibility, fixes duplication of posts, fixes users connections

Compatibility fixes are from @baden03

I modified the way the plugin checks for other plugins being activated. This modification does not rely anymore on the dependent plugins to be installed in a specifically named folder.

Then, in order to fix duplicated posts, I noticed that the duplicate ajax action was done at the init hook by WPML, but P2P inits itself later (at the wp_loaded hook). I so implemented a little hack to init P2P and P2P-WPML earlier in case of a WPML Ajax request.

Lastly, users connections synchronization was buggy because the plugin was looking for translated post to users using user's IDs ... I fixed it.