[php mode] Add support for interpolated variables within double-quoted strings.

I have added support for interpolated variables within double-quoted strings.
### Example
![php_mode](https://cloud.githubusercontent.com/assets/5851313/2712257/6741fb74-c4d8-11e3-8a9e-3ab012ce639f.png)

I have tried to mimic rules described here:
https://github.com/php/php-src/blob/master/Zend/zend_language_parser.y#L1234
(see encaps_list)

_TODO_:
The same rules of highlighting should apply also php's HEREDOC blocks.