Useful local variables in *stack* frames are removed

[tahajahangir]

When using auto_log_stacks=True, frames are pruned by slim_frame_data and only local-variables of top/bottom 12 frames are kept. This is because in_app are not calculated for stack frames and include_paths are ignored.

When using auto_log_stacks bottom frames are usually in raven or logging package, and top frames are usually framework frames (like uwsgi or django), and this results in loss of useful information.

Also it's a good idea to add configuration to change the frame_allowance, this is hard-coded to 25 in the code and is not configurable.