Use getClass().getResource() instead of URL to get certificate.

Following on from pull request [14](https://github.com/jayway/rest-assured/pull/14) this makes specifying the location of a certificate a bit nicer.

    scheme.setCertPath("/soapui.p12");

Instead of

     scheme.setCertURL("file:///my/project/src/test/resources/soapui.p12");