Add Behat tests

Added some behat test covering pickle. Four suites have been created:

* Pickle source (`pickle`)
* Pickle Phar (`pickle_phar`)
* PECL extensions conversion with Pickle source (`pecl`)
* PECL extensions conversion with Pickle Phar (`phar_pecl`)
please don't depend on ``*`` but on ``~3.0`` instead so that things does not break the day Behat decides to ship a new major version (which is not planned for now but anyway)