Added feature that allows Camel header to set Process initiator

I added some code to the activiti-camel project that allows a Camel header to be specified, whose value will be used when setting the Initiator on the workflow instance.

So with this you can have a Camel route like below:

```java
      from("direct:startWithInitiatorHeader")
          .setHeader("CamelProcessInitiatorHeader", constant("kermit"))
          .to("activiti:InitiatorCamelCallProcess?processInitiatorHeaderName=CamelProcessInitiatorHeader");
```

and "kermit" will be set as the initiator of the workflow instance.