[WIP] make scrapy command line utility faster

I noticed that `scrapy` command-line utility became much slower during last half year or so. This is an attempt (unsuccessful so far) to fix that by deferring imports that cause the slowdown (see https://github.com/pyca/pyopenssl/issues/137). On my machine this fix makes ``scrapy`` without arguments to run in 0.3s instead of 0.8s, but all other commands (e.g. ``scrapy list``) still take > 0.8s because they indirectly import OpenSSL (through twisted).