Minor trans-related changes for jobs.
Remove a couple unused references on the job execution path through the code and require 'less' from the object to run jobs (i.e. try to use only trans.user and not trans.get_user() and make access to the underlying session optional in galaxy.tools.actions).
Extract some mixin's out of trans allowing trans-like functionilty outside web threads...
In particular break out mixins defining context-based utilities for interacting with app, an user, and a history. In downstream work on workflow scheduling this proves nessecary and sufficient to create an context for 'execute'-ing tools (create jobs) outside of web threads in response to workflow requests.

Unit tests for some of this.