Expression Language support in EvaluateXPath

As a user, I would like support for Expression Language in the EvaluateXPath processor.

This would be a nice modification as it would allow passing of XPaths from attributes or content, and would permit more generic flows/templates.

As discussed in http://apache-nifi-developer-list.39713.n7.nabble.com/Expression-language-support-in-EvaluateXPath-td16441.html, ideally we want to keep the actual performances for the case when we don't use expression language, and we compile expressions at trigger time only where expression languages are used.