new api (#5)

* GenericContainer added

* DockerContainer additional methods

* mysql environ variables added

* mysql environ variables added

* test for env variables added

* test for env variables fixed

* postgres fix

* postgres get_connection_url fix

* release process description changed

* with_name added to docker container class

* with_mount_volumes added to docker container

* test fix for port bindings

* with_exposed_ports added

* _configure deleted from DockerContainer class

* tests fix

* test fix for custom image