Enhancements to camel-splunk and camel-splunk-hec

Couple of small enhancements to camel-splunk and camel-splunk-hec

• Explicit documentation about the token in camel-splunk-hec is not the user's token, but the token generated for the HEC.
• Add support for token authentication, a safe alternative to the user's password.
• Escape the error cause when authenticating to splunk (original stacktrace is lost)
• Document about using the realtime consumer.