add a thread decorator

[tpatel]

• the decorator doesn't create a step, it uses a context to set the current thread_id
• the steps use the context to get the thread_id
• the priority is: 1/ thread_id from the parent step 2/ thread_id from the context (set by thread decorator) 3/ thread_id from the argument
• this decorator isn't mandatory, as one can set the thread_id in each step
• can be used with a parenthesis or not (uses a hack to force keyword argument and detect whether the decorator is used with parenthesis or not)
• sets a default thread_id if it isn't set
• introduces a new way to get the current thread id