improve cost of instrumentation by up to 15x in chrome

Note: This is only enabled if someone engages the inspector, or manually enables instrumentation.
Instrumentation with stack is now opt-in

To improve performance, the default is now not to track stack with
promises.

To include the stack in instrumentation:
`RSVP.configure('instrument-with-stack', true);`
Merge pull request #304 from teddyzeenny/stack-opt-in

Instrumentation with stack is now opt-in
Merge pull request #303 from tildeio/faster-instrumentation

improve cost of instrumentation by up to 15x in chrome