bugfix: "--- error_log" should work on each test request's own logs only.
feature: added new sections "--- grep_error_log" and "--- grep_error_log_out" (for #6).
feature: added utility function check_accum_error_log for checking the accumulated error logs in "--- error_log" and "--- no_error_log".
documented the "--- grep_error_log" and "--- grep_error_log_out" sections. also included some minor fixes.
documented the "check_accum_error_log" function.
Merge remote-tracking branch 'upstream/master'

Conflicts:
	lib/Test/Nginx/Socket.pm
	lib/Test/Nginx/Util.pm