Update wkhtmltopdf binary and update README.rst

resume:

* update wkhtmltopdf binary to 0.12.0 version on before_script.sh
* update docs to reference a wkhtmltopdf on github ( https://github.com/wkhtmltopdf/wkhtmltopdf )
* add link to official site http://wkhtmltopdf.org/
* move tests from Makefile to run_tests.sh


fix wrong pull request #44