Let Jetty find a free port by itself

Send in 0 as the port when user has not specified a port. This makes
Jetty choose a random port.

To make sure the port Jetty has chosen is not taken, catch any eventual
BindExceptions and retry 2 times.

This closes #153.
Merge branch 'jetty_find_and_bind_to_free_port' of https://github.com/finn-no/rest-driver into finn-no-jetty_find_and_bind_to_free_port

Conflicts:
	rest-client-driver/src/main/java/com/github/restdriver/clientdriver/ClientDriver.java