avoid download large response

introduce DOWNLOAD_MAXSIZE and DOWNLOAD_WARNSIZE in settings and download_maxsize/download_warnsize in spider/request meta, so downloader stop download as soon as the received data exceed the limit. also check the twisted response's length in advance to stop download as early as possible.

NB, this PR redone the changes in https://github.com/scrapy/scrapy/pull/336
According to Wikipedia, Mb is used as a shortcut for [Megabit](http://en.wikipedia.org/wiki/Megabit) while MB is used as a shortcut for [Megabyte](http://en.wikipedia.org/wiki/Megabyte)