XPath expression - Add option to use Saxon, so you can configure this from XML DSL

See nabble
http://camel.465427.n5.nabble.com/XPath-Language-Saxon-for-XPath-2-0-on-Spring-DSL-tp5056545p5056545.html

We should add an option so you can configure to use saxon, like you can do in Java DSL.
Something alike:

<xpath saxon="true">/foo/bar</xpath>