Camel body and headers to Activiti variables

Added more functionality to ActivitiCamel component.

* Property copyVariablesFromHeader: Allows Activiti Producer to read all the Camel headers as variables. It is useful for some endpoints such as Email, HTTP or JMS.
* Property copyCamelBodyToBodyAsString: Allows passing the Camel body as a String, using CamelContext's type converters from Activiti Producer
* When the Camel Body is not a Map&lt;String, Object&gt; the object contained is added as _camelBody_ variable