sender 1.5.7

Adds :__sender__ and :__caller__ to the built-in :__callee__ and :__method__ methods in Ruby 1.9.1. Also provides object-oriented :backtrace supporting n-levels backward, :each_backtrace_frame for iteration, :backtrace_includes?, and :backtrace_includes_one_of? for context inspection, and :backtrace_frame_with and :backtrace_frames_with, which return matching frame information for the frame(s) matching the given description.