Use class loading only to detect JSF version

The JsfApiAbstraction attempts to detect the presence of the method FacesContext#isPostback() to determine if JSF 2.0 is on the classpath. This causes a problem if Sun's stripped APIs are on the classpath at runtime, even if the developer has no intention of using JSF. When an attempt is made to detect the presence of a method from stripped code, the check blows up.

An example of the stripped code is the artifact javax.javaee-api:6.0 in the NetBeans Maven repository: http://download.java.net/maven/2/javax/javaee-api/6.0/

It would be safer to search for the javax.faces.component.behavior.Behavior class to detect if JSF 2 is present.