* added Twisted log observer for Scrapy, with unittests
* use numeric values from Python logging module for log levels
* removed scrapy.log.exc() function - use scrapy.log.err() instead
* removed logmessage_received signal - write a (twisted) log observer instead
* dropped support for obsolete `domain` argument
* dropped support for old setting names: LOGLEVEL, LOGFILE (replaced by LOG_LEVEL, LOG_FILE)
* deprecated `component` argument