Semantic Integration

This is a long-term goal, but I would like to integrate [Semantic support for PHP](https://github.com/trashofmasters/.emacs.d/tree/semantic-php) by Andrea Turso, @trashofmasters. I have spoken with him and we agreed that first the functionality should be tested by publishing it as a separate package. But once everyone feels that it is stable then I would like to merge it into PHP Mode, because I believe it will bring a lot of useful functionality, such as Intellisense-ish features for PHP programming. This issue is for discussing the pros and cons of integrating the Semantic PHP code and anything we need to keep an eye on down the road for that merge in the future---assuming it is feature that we want to merge into PHP Mode.