Use funcsummary.txt

A quick and simple implementation of #7.  It seems to work.  What do you think?

Because I used `php-mode-syntax-table` to grab the symbol at point, I added `php-mode` to the Carton file.  That's probably overkill, so I can remove the dependency if wanted. (Eventually it would be ideal for `php-extras` to be folded into `php-mode` anyway, I think.)