Make it compile and run

Hi there,

Made this work again, see http://dotty.herokuapp.com/.

Changes:

* use default php buildpack from heroku (closes issue #15)
* use updated buildpack for graphviz that does not include -lperl
* add README file
* remove .png option (because current graphviz version does not support it)
* typo in example
* add clearcache option (should be made optional)