Add support for company-mode

Company-mode is similar to autocomplete-mode, but included in GNU ELPA (and possibly to be included in a future release of Emacs? Not sure.)  This patch adds a simple company-mode backend using the php-extras cache of function metadata.  It annotates the completions with their arguments, and shows short descriptions in the echo area.  It can also pop up a documentation buffer with the longer documentation -- not shown in the screenshot.

![screen shot 2014-03-18 at 23 15 02](https://f.cloud.github.com/assets/1049744/2454871/a128d042-aef3-11e3-9626-82d71b474b74.png)