Fix st2auth entry point.

We still want to allow user to run the st2auth as a standalone service for
testing purposes.
Improve jsexpose decorator.

If method expects a body and takes additional argument in the signature, pass
this argument to the method even if no body is provided.
Merge pull request #819 from StackStorm/bug-fixes/STORM-858/improve_jsonexpose_body_handling

Improve jsexpose body handling