Trace exception local variables and args per frame

New configuration options and defaults
config.locals.enabled = off
config.locals.max_trace_frames = 100

Turned off by default because this may not be performant just yet!

  1. Stores each Ruby process' local vars and method arguments
  2. When gathering exception data, "replay" over each frame to get the variables for that frame's context
  3. Dump the cache and repeat
This could be resolved if instead of having a `Rollbar` module doing all the stuff, the `Rollbar` methods just instanciate a `Notifier` object with all the logic that actually resides in the singleton `Rollbar`. 

This change will become Rollbar to be threadsafe.