add php-parser

* [x] add gadget
* [x] ~~test: `@todo`~~ see #38
* [x] test: var_dump
* [x] test: die
* [x] ~~test: `// dont commit`~~ see #38 
* [x] test echo

more complex ideas

* [x] blacklist of functions that are not allowed (careful with language constructs vs. functions)
* [x] ~~blacklist of comments that trigger issues~~ see #38 
* [x] issue severity should be configurable per blacklist comment/function

Question: how to escalate on `goto` :smile: 
Also move to parent class? I would like to use it in the comment_commenter Gadget.